RFR: 8241976: Add test for GCPhaseConcurrentLevel1 JFR event

[Stefan Karlsson]

Hi all,

Please review this patch to add a test for the new 
GCPhaseConcurrentLevel1 JFR event.

https://cr.openjdk.java.net/~stefank/8241976/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8241976

This also fixes the test failure in 
test/jdk/jdk/jfr/event/metadata/TestLookForUntestedEvents.java, where 
the test complains that the JFR event isn't tested. I'd like to push 
this patch ASAP to fix that failure.

Thanks,
StefanK