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>

in favour of Constants.ClassFileVersion.of(int, int). The few ones
retained are used from production code, the vast majority of removed
ones only from test code.

This gets rid of yet another '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>

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>

- Java 14 feature sample classes moved from 'bugs' to 'features'
- One test case using a Java 14 preview feature was moved to the
  Java 14-only tests

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

Some Ajc196 tests are using preview features (see .../ajc196.xml), i.e.
they will fail on Java 15+ because code compiled with '--enable-preview'
can only run on the same JVM version, not on a more recent one. Hence,
the preview-using tests are now being excluded in order to make the
build run on Java 15, even though no Java 15 features are present in the
current 1.9.7 snapshot.

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