RFR: 8231943: ZGC: Enable serviceability/dcmd/gc/RunGCTest

[Per Liden]

This test is currently disabled for ZGC, but it can easily be enabled by 
adjusting the expected log string. ZGC doesn't print "Pause Full", but 
it still prints the "(Diagnostic Command)" part.

Also, the test enables gc=debug logging, which is unnecessary since this 
is always printed on the gc=info level.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231943
Webrev: http://cr.openjdk.java.net/~pliden/8231943/webrev.0

Testing: Manually ran test with all GCs (except Epsilon)

/Per