RFR: Clean up Shenandoah configurations in shared tests

[Roman Kennke]

So we have $TEST_VM_OPTS to run any non-GC-specific test with
Shenandoah, this should cover a lot of ground here.

What would happen if one tried to run a test that has
-XX:+UseShenandoahGC (e.g. next to UseCMS) in a build without
Shenandoah? Is there a way to predicate tests to only run in this config
if the config is actually present?

Roman

> 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
> 
>