RFR: 8236106: [TESTBUG] Shenandoah: Make TestThreadFailure more resilient

[Roman Kennke]

Currently, we see frequent timeouts with TestThreadFailure. On my local 
machine, this test standalone can run 1:30, on CI box it can exceed the 
8minutes limit. Problem seems to be that it's running into 
back-to-back-GCs but not into OOM fast enough.

Decreasing the max heapsize of the launched process seems to complete 
much faster (in under 2 minutes on the CI box) and thus more resilient.

http://cr.openjdk.java.net/~rkennke/JDK-8236106/webrev.00/

Testing: hotspot_gc_shenandoah, several runs of the test standalone

Ok?