RFR 8068260: java/io/Serializable/clearHandleTable/ClearHandleTable.java timed out

[Chris Hegarty]

This is a small, test only, review request to fix an intermittently 
failing test.

It replaces a "bad" technique, heap exhaustion, with the "less bad" 
technique of calling System.gc, potentially multiple times, to clear 
weak references. With this change the test runs much quicker, and has 
not failed in several thousands of runs on a local machine.

http://cr.openjdk.java.net/~chegar/8068260/webrev.00/webrev/

-Chris.