RFR (S) 8208352: Merge HeapMonitorTest and HeapMonitorGCTest code

[JC Beyler]

Hi all,

Two of the tests shared code and we could refactor to limit future
fixes/changes in both spots. Here is the small webrev that does that:

Webrev: http://cr.openjdk.java.net/~jcbeyler/8208352/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8208352

Thank you for the future reviews,
Jc
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