Invalidate Caches / Restart can also help "no tests found in the package"

A couple students received "no tests found in the package" errors from TestNG in Android Studio even though the tests could compile successfully. Everything seemed to be in order, and an abridged version of the test results even appeared in the [build] tab. I asked the students to try a Gradle sync and a File | Invalidate Caches / Restart and that appeared to fix it. In general, mysterious problems tend to go away with a resync or cache clear.