RFR: 8236153: ZGC: gc/z/TestUncommit.java fails with java.lang.Exception: Uncommitted too fast

[Per Liden]

The test gc/z/TestUncommit.java fails now and then on Windows when using 
-Xcomp. This test can fail if it's severely starved on CPU, as it will 
cause the timing to be off. The logs confirms that the test took an 
unusually long time to execute, suggesting it was starved on CPU. This 
only happens in test tiers using -Xcomp, which is likely causing the 
unusually high load. This patch disables this test when using -Xcomp. 
I've enabled some GC logging, which should helpful if this test ever 
fails again.

Bug: https://bugs.openjdk.java.net/browse/JDK-8236153
Webrev: http://cr.openjdk.java.net/~pliden/8236153/webrev.0

/Per