libsync: tests: redefine PollOnDestroyedTimeline()

On mainline if the sw_sync timeline is destroyed the fences doesn't not
signal or error. So change the test to check if the fence is still there
by polling the fence with timeout zero and asserting if it is not
signalled.

Test: Sync unit tests still passes.
Change-Id: Icb8e629018eef35074ae91d0f29ed1f12e90492b