RFR: 8157452: [TESTBUG] PLAB tests don't handle unexpected GC

[Michail Chernov]

Hi,

Could I have a couple of reviews for this change please?

https://bugs.openjdk.java.net/browse/JDK-8157452
http://cr.openjdk.java.net/~mchernov/8157452/webrev.00/

This change improves handling of unexpected GC during testing.
Both PLAB tests expect to find in log only WhiteBox initiated GC.

Testing was performed on different configurations.

Thanks,
Michail