Do not run tests using Java 14 preview features on this Java 15 compiler

Do not run tests using Java 14 preview features on this Java 15 compiler

These tests need a Java 14 level AspectJ compiler, because they use
version-specific preview features. This compiler has been upgraded
to a Java 15 compliant JDT Core already, i.e. it does not support
preview features of a previous version anymore.

An error message similar to the above explanation will appear when
trying to run any XMLBasedAjcTestCaseForJava14Only subclass, such as
Ajc196PreviewFeaturesTests (currently the only one).

When running AllTestsAspectJ196, Ajc196PreviewFeaturesTests will not be
added to the test suite anymore.

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