For now, only the "pattern matching for switch" tests from previews 1
and 2 were adjusted to work in preview 3, because guarded patterns were
replaced by 'when' clauses in 'switch' blocks. Therefore, existing test
classes did not compile anymore and had to be replaced by syntactically
upgraded versions with content merged from preview 1 and 2 classes.

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

- Fix one fault sanity test configuration
- Deactivate Java 16 preview tests (no longer supported by Java 17
  compiler)
- Test sealed classes as final on Java 17 (no longer preview)
- Add tests for JEP 406, pattern matching for switch (preview). At
  present, the beta 17 branch of JDT Core does not handle the tested
  features and expected compile errors correctly yet, so I had to
  temporarily deactivate test execution, only printing TODO messages.

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

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

- 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>

Also fix some minor details in Java 14 suite

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