RFR [8u]: Make sure -verbose:gc, PrintGC, PrintGCDetails work consistently

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/8u-logging/webrev.01/

This is 8u-specific patch, because we have this logging compatibility layer there. We need to make
sure -verbose:gc, PrintGC, PrintGCDetails work consistently there. Adds a test. Fixes another one.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey