rerun-fails: run all failed tests in a package at once

When I first wrote `--rerun-fails` there were limitations with the `go test -run` flag that required running each test individually (https://github.com/golang/go/issues/39904). I think now that https://github.com/golang/go/commit/025308fe084264538f49924b3f52d8d6b6359658 is done (and has been in all Go versions since go1.18), this can be fixed to re-run all the tests in a package at once.

Start with the implementation of the --rerun-fails option and review the current use of the go test -run flag. Confirm the Go 1.18 behavior described in the linked Go issue and commit, then verify that all failed tests in a package are rerun together rather than individually.

Written by the indexing model from the issue text.