RFR: Fix shared tests to run without Shenandoah

[Roman Kennke]

Several shared tests wouldn't run or fail when Shenandoah is not
available. This fixes them. Some needed an extra test section, some
needed a runtime test, and one I did not know what to do:

test/hotspot/jtreg/gc/TestFullGCCount.java

http://cr.openjdk.java.net/~rkennke/fix-shared-tests/webrev.00/

Testing: hotspot_gc_shenandoah, hotspot_gc with -shenandoahgc build

OK?