Fix go/vet composite errors for signer_test.go

Error: packager/signer/signer_test.go:167:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:168:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:220:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:221:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:268:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:269:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:286:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:287:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:301:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:302:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:311:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:312:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:321:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:330:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:343:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:357:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:372:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:386:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:402:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:419:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:436:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:479:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:495:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:516:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:540:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:563:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:586:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:603:11: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:614:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:632:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:652:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:665:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:677:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:692:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:704:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:721:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:738:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:756:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:775:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:793:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:810:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:827:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:848:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:905:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:940:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:1041:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:1064:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:1077:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
Error: packager/signer/signer_test.go:1148:10: github.com/ampproject/amppackager/packager/util.URLPattern composite literal uses unkeyed fields
# github.com/ampproject/amppackager/transformer
Error: transformer/transformer_test.go:314:49: call of t.Fatalf copies lock value: github.com/ampproject/amppackager/transformer/request.Request contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex

Start with packager/signer/signer_test.go at the reported lines and transformer/transformer_test.go:314, then run go vet on the affected packages. Address the reported URLPattern composite literals and copied-lock error while preserving test behavior; done means go vet reports none of these errors.

Written by the indexing model from the issue text.