Make EnableCollection using tests more robust

The EnableCollection and the IsCollected tests are somewhat flakey
since they don't check that a GC has actually had time to happen. We
fix this by making them both all allocate a marker object that they
will wait to be collected before continuing the test, meaning that at
least one GC has occurred.  This also unifies the 3 different
(incorrect) ways the tests tried to force a GC.

Test: while ./art/tools/run-libjdwp-tests.sh --mode=host --test org.apache.harmony.jpda.tests.jdwp.MultiSession.EnableCollectionTest; do; done;
Test: while ./art/tools/run-libjdwp-tests.sh --mode=host --test org.apache.harmony.jpda.tests.jdwp.ObjectReference.EnableCollectionTest; do; done;
Test: while ./art/tools/run-libjdwp-tests.sh --mode=host --test org.apache.harmony.jpda.tests.jdwp.ObjectReference.IsCollectedTest; do; done;

Bug: 70958370
Bug: 69121056

Change-Id: I4f49c4fa4b9105fc1c2a66c9299407262ab6141f