TestNG "cannot find class in classpath" can mean the tests didn't compile

I've recently been trying to help students troubleshoot issues with running TestNG tests in Android Studio. A common error is "cannot find class in classpath", referring to the class containing the tests. This often indicates that there's a problem causing the compilation of the test suite to fail. More specific error messages might be visible on the [build] tab of the lower pane where the test results appear.