Re-enable EnableCollection/IsCollected tests

Rationale:
A few refinements have hopefully removed the flakiness.
Repeating the following three tests came out clean.

Test: org.apache.harmony.jpda.tests.jdwp.MultiSession.EnableCollectionTest
Test: org.apache.harmony.jpda.tests.jdwp.ObjectReference.EnableCollectionTest
Test: org.apache.harmony.jpda.tests.jdwp.ObjectReference.IsCollectedTest

Bug: 70958370
Bug: 69121056
Change-Id: Ic7230c521464a085bdaec242c8cde507a3e83d1d