It's quite often situation when tests fail because run with inappropriate GC: tests which explicitly require g1 are executed with parallel GC.
Once CODETOOLS-7900307 is fixed such tests could define the @requires tag to avoid running with a wrong GC.

As temporary and easy solution of GC flag conflicts I suggest introducing a few new jtreg groups:

requires_g1gc
requires_parallelgc
requires_serialgc
requires_cmsgc
requires_gc
not_requires_gc

Those who need to execute tests with parallel GC could give the following command:
% jtreg test/:not_requires_gc

Note, this fix is going to be a temporary solution. If a test belong to the requires_parallelgc group, it doesn't mean that it could be executed with Parallel GC... The test may use G1 gc...
The @requires tag should fix the problem better.