RFR: 8231489: GC watermark_0_1 failed due to "metaspace.gc.Fault: GC has happened too rare"

[Per Liden]

vmTestbase/metaspace/gc/HighWaterMarkTest relies on timing and fails 
when "Metaspace GC Threshold" isn't handled in a STW pause.

The problem can be reproduced on both G1 and ZGC, but it's hard, as the 
window is small. However, it reproduces every time when injecting a 
100ms delay to prolong the GC cycle a bit. This test used to be disabled 
for G1 with ClassUnloadingWithConcurrentMark, but JDK-8204163 enabled it 
about a year ago.

Fixing the test properly is tricky. As far as I can see, we can either:
1) Disable this test for G1+ClassUnloadingWithConcurrentMark and ZGC, or
2) Add a sleep in the test loop, to make the race less likely to happen, or
3) Remove the test completely, with the rational that it's a buggy low 
value test.

I've gone with 1) here. The test is already disabled for CMS today, with 
code in the test itself (i.e. not using @requires), so I did two 
alternative patches:

A) Follows the existing style to disable the other GCs:
http://cr.openjdk.java.net/~pliden/8231489/webrev.0-alt1

B) Adds @requires to the tests using the HighWaterMarkTest class, and 
removes the old check to disable CMS:
http://cr.openjdk.java.net/~pliden/8231489/webrev.0-alt2

I prefer B, but I don't have a strong opinion on which way to go.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231489

/Per