Make sure Java 14-only tests using preview are not executed on JVM 15+

Make sure Java 14-only tests using preview are not executed on JVM 15+

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>