TestNG results in Android Studio can be stale if there are build failures

One student asked about a puzzling failure in a TestNG test in Android Studio. Their code seemed correct and in fact worked on my machine. Apparently there was a compilation failure somewhere in the project that of course prevented a new executable from being generated, causing the test suite to test an outdated version. Build | Rebuild Project (to do a recompile and show the Build tab) helped identify the problem.