Re-enable disabled VG test.
Properly lock the shared timer offset.

The timer offset used by cwtest_advance_time_ms is accessed from
multiple threads. Ensure we lock it properly.
Fix the ExpiredChain UT failure.

Problem was that the first poll was taking it from Completed to
Terminated, and we needed a second poll to take it from Terminated
to Destroyed. Make poll() do repeated polls until quiescent.
Merge pull request #39 from Metaswitch/ut-robustness

[Reviewer: Matt] UT robustness