G1 can now unload classes after concurrent marking, so this tests incorrectly fails when it doesn't find 20 Full GCs.

This applies to the following tests:
RULE metaspace/gc/watermark_0_1 Exception metaspace.gc.Fault: GC has happened too rare: ... times, expected count greater than ...
RULE metaspace/gc/watermark_10_20 Exception metaspace.gc.Fault: GC has happened ... times, approximate count is ... to ...
RULE metaspace/gc/watermark_70_80 Exception metaspace.gc.Fault: GC has happened ... times, approximate count is ... to ...