RFR (S): 8026047: add regression test for DisableExplicitGC flag

[Filipp Zhinkin]

Hi,

I would like to get couple reviews on regression test for 
DisableExplicitGC flag.

It verifies that System.gc() calls did not trigger garbage collection if 
DisableExplicitGC is turned on and vice versa.

Bug ID:  https://bugs.openjdk.java.net/browse/JDK-8026047
Webrev:  http://cr.openjdk.java.net/~kshefov/8026047/webrev.00/
Testing: manual on local host, automated on various platforms.

Thanks,
Filipp.