RFR: Clean up Shenandoah configurations in shared tests

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/cleanups-12/webrev.01/

This revises the changes we have in shared tests:
  *) When test does not have specific GCs already, it expects to run a single GC configuration that
can be overriden globally. We have no business in peddling UseShenandoahGC there;
  *) When tests have separate configuration for CMS, we need to have separate configuration for
Shenandoah, so that test results for CMS are not contaminated with Shenandoah failures;
  *) Minor accidental differences against upstream accumulated over multiple merges;

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey