RFR: 8240714: ZGC: TestSmallHeap.java failed due to OutOfMemoryError

[Per Liden]

The gc/z/TestSmallHeap.java test failed once due to OutOfMemoryError. 
When using a 8M heap, this test is fairly sensitive in the sense that 
the heap will be very crowded and the heap headroom is small. When 
running as "main/othervm" there are additional jtreg threads running in 
the VM. These threads can apparently (sometimes?) allocate enough memory 
to disturb the test itself, pushing it over the edge with OOME as a 
result. To avoid having these threads running in the same VM as the test 
itself I've adjusted the test to spawn a new test VM through ProcessTools.

Webrev: http://cr.openjdk.java.net/~pliden/8240714/webrev.0
Bug: https://bugs.openjdk.java.net/browse/JDK-8240714

Testing: Manual

cheers,
Per