RFR(S): 8150865: SQE test: GC unified logging: check that dynamic log level doesn't break anything

[Kirill Zhaldybin]

Dear all,

Could you please review this fix for 8150865?

The test creates two types of threads: the first type stresses gc and 
the second type switches gc unified loggingl level/file output.
The test is a stress test and it is expected that it finishes normally 
(no oom or crash).

WebRev: http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8150865/webrev.00/
CR: https://bugs.openjdk.java.net/browse/JDK-8150865

Thank you.

Regards, Kirill