Replaces now obsolete base classes
  - XMLBasedAjcTestCaseForJava[n]OrLater,
  - XMLBasedAjcTestCaseForJava[n]Only.

The new class is parametrised with minimum and maximum Java version and
hence can replace all the other classes. This does not only apply the
DRY principle, but also makes adding tests for new Java versions less
tedious.

By chance, I also noticed missing sanity tests for Java 12, which I
added as a little drive-by benefit.

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

Replace them by a uniform method 'isVMGreaterOrEqual(double)', also
overloaded for int.

This gets rid of one 'AspectJ_JDK_Update' tag. One less place to check
and update with each newly supported Java version. :-)

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

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

This was required by the Eclipse team as one precondition for the next
release.

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>

	  	AJC produces partial output jar file, when there are warnings during weaving
(new dependencies)

ensure messages are associated with source wherever possible