RFR (S) 8247757: Shenandoah: split heavy tests by heuristics to improve parallelism

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8247757

There are plenty of tests that take a while and test multiple modes and heuristics. Their
configurations are split by mode. Splitting them further by heuristics improve their parallelism
even more.

Fix:
  https://cr.openjdk.java.net/~shade/8247757/webrev.01/

Testing: hotspot_gc_shenandoah (even faster now)

-- 
Thanks,
-Aleksey