RFR: 8026047: [TESTBUG] add regression test for DisableExplicitGC flag

[Michail Chernov]

Hi all,

Please review the fix with new test for DisableExplicitGC VM flag.

Webrev: http://cr.openjdk.java.net/~eistepan/~mchernov/8026047/webrev.00/
Enhancement: https://bugs.openjdk.java.net/browse/JDK-8026047

There is one scenario with 6 parameters combinations.

1,2,3 scenarios test default value for DisableExplicitGC, 
DisableExplicitGC=true and DisableExplicitGC=false
4,5,6 scenarios check how VM works when VM changes DisableExplicitGC 
flag using WhiteBox.

Test tries to call System.gc() and check that VM puts message to stdout. 
After (in case of 4,5,6 scenarios) test tries to change 
DisableExplicitGC value and calls System.gc() twice.

Test was executed locally on linux-i586 with all available GC and 
several GC-related flags. Also it was executed using Aurora on other 
platforms.

Thanks,
Michail