RFR:8147847:serviceability/tmtools/jstat tests are failing with -XX:+ExplicitGCInvokesConcurrent

[Alexander Kulyakhtin]

Hi,

Could you, please, review this trivial tests-only fix:

CR: https://bugs.openjdk.java.net/browse/JDK-8147847 "serviceability/tmtools/jstat tests are failing with -XX:+ExplicitGCInvokesConcurrent"
Webrev: http://cr.openjdk.java.net/~akulyakh/8147847/index.html

The changed tests compare the output from the "jstat -gc" (or from similar commands) before and after an explicit garbage collection has taken place.
Therefore, the tests should not be executed if the option -XX:+ExplicitGCInvokesConcurrent is set.
The fix uses the @requires tag to make sure this is the case.

Best regards,
Alexander