Add serviceability_ttf_virtual group to exclude jvmti tests developed for virtual threads

The JTREG_TEST_THREAD_FACTORY=Virtual mode is not compatible with some jvmti tests developed to test virtual threads.

So it would be easier just to add a group which exclude those tests

serviceability_ttf = \
  serviceability/ \
  -serviceability/jvmti/vthread \
  -serviceability/jvmti/thread \
  -serviceability/jvmti/events \
  -serviceability/jvmti/negative.