tailscaleroot: TestGoMod from go_mod_test.go failure message does not identify which replace directive triggered it

TestGoMod in go_mod_test.go checks that go.mod has zero replace directives; however, if the test fails, it only reports the count of replace directives, it doesn't say which module/s is/are being replaced.

Fix idea is to have the test iterate over f.Replace and print each offending module (old path/version => new path/version); after that, print the existing count.

This is not a bug. This is a small change; it doesn't alter test behavior, it just makes the failure message a little bit clearer :D

Start in go_mod_test.go at TestGoMod and inspect how f.Replace and the current replace-directive count are reported. Update the failure output to identify each old and new module path/version, then retain the existing count and run the relevant Go test to confirm the diagnostic is clear.

Written by the indexing model from the issue text.