RFR: 8225084: gc/z/TestHighUsage.java fails with unexpected allocation stall

[Per Liden]

TestHighUsage might fail in case the test system is starved on CPU. This 
patch adds a delay to allow for a GC cycle to complete even in those 
situations. Also adding gc+start logging to allow for easier debugging 
of test failures.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225084
Webrev: http://cr.openjdk.java.net/~pliden/8225084/webrev.0

/Per