RFR: 8166719: gc/stress/TestStressG1Humongous.java fails with OOME

[Michail Chernov]

Hello,

Could I have a reviews for this small fix?
https://bugs.openjdk.java.net/browse/JDK-8166719
http://cr.openjdk.java.net/~mchernov/8166719/webrev.00/
OOME happened in ThreadPoolExecutor. To avoid this test was updated to 
not to use j.u.c.Executors, just j.l.Thread.
Tested on different hosts with set of nightly flags - test passed.

Thanks,
Michail