fix a race is the LLDB test suite results collection

The race boiled down to this:

If a test worker queue is able to run the test inferior and
clean up before the dosep.py listener socket is spun up, and
the worker queue is the last one (as would be the case when
there's only one test rerunning in the rerun queue), then
the test suite will exit the main loop before having a chance
to process any test events coming from the test inferior or
the worker queue job control.

I found this race to be far more likely on fast hardware.
Our Linux CI is one such example.  While it will show
up primarily during meta test events generated by
a worker thread when a test inferior times out or
exits with an exceptional exit (e.g. seg fault), it only
requires that the OS takes longer to hook up the
listener socket than it takes for the final test inferior
and worker thread to shut down.

See:
http://reviews.llvm.org/D19214

reviewed by:
Pavel Labath

llvm-svn: 266624