RFR(T) : 8244491 : make runtime/cds/appcds/TestZGCWithCDS.java test more robust

[Igor Ignatyev]

http://cr.openjdk.java.net/~iignatyev//8244491/webrev.00
> 30 lines changed: 4 ins; 20 del; 6 mod;


Hi all,

could you please review this patch for TestZGCWithCDS.java test?
from JBS:
> runtime/cds/appcds/TestZGCWithCDS.java test makes assumptions about default GC behavior, in some cases, this might lead to false-positive results, so it's better to explicitly select GC whose behavior satisfies the needs of the test.
the patch updates the test to explicitly set GC in all spawned JVMs, in cases where we used to use a default GC, we now use SerialGC. this effectively made 'Run with ZGC turned off' the equivalent of 'Run with +UseCompressedOops +UseCompressedClassPointers', so the former got removed.

JBS: https://bugs.openjdk.java.net/browse/JDK-8244491
webrev: http://cr.openjdk.java.net/~iignatyev//8244491/webrev.00
testing: TestZGCWithCDS.java test

Thanks,
-- Igor