Add Java 16 test suite for AspectJ 1.9.7 + test refactorings

Add Java 16 test suite for AspectJ 1.9.7 + test refactorings

- Test all features which were preview in 14+15 and are now final in 16,
  compiling them with language level 16.
- For Java 15 we only have sanity tests (and of course the Java <14
  tests), compiling Java 16 features to target 15 does not seem to work.
- Test remaining Java 16 preview feature (sealed classes).
- Instead of overriding runTest(String) in several base classes like
  XMLBasedAjcTestCaseForJava*Only or XMLBasedAjcTestCaseForJava*OrLater,
  we now override setUp() from JUnit's TestCase base class. This will
  run before runTest(String) and make the tests fail much faster, if a
  user tries to run them on the wrong VM.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>