Add test cases where we start three worker threads, with a write watchpoint...

Add test cases where we start three worker threads, with a write watchpoint set.  As soon as a watchpoint is hit in either of the worker thread,
we delete the watchpoint.  The test succeeds when no more watchpoint hit event fires after the deletion of the watchpoint.

related to rdar://problem/11320188

llvm-svn: 157084