Same symptom as #63844 on the master branch: there are leaked goroutines when the test ends.

This is not surprising because the test contains an unlimited call to ReportDiagnostics(). If the reg server is unavailable, that call blocks asynchronously and the goroutine leaks.

This should be fixed by making the test impervious to reg server failures, by using a temporary in-memory collector instead and customizing the TestingKnobs.OverrideReportingURL field.