When run with `ShenandoahGCMode=generational` TestChurnNotifications fails in certain modes. One solution is to explicitly mark non-generational mode tests to state that they should not run if generational mode is explicitly set. While it is not clear to this author if the other ShenandoahGCHeuristics settings make sense with generational mode, if those are tested then the mem.pool system property should be set. It would, however, be ideal if the test would use the right memory pool to use without having to specify it in the test.