[lit] Enqueue tests on a separate thread to not hit limits on parallel queues

Summary:
The multiprocessing.Queue.put() call can hang if we try queueing all the
tests before starting to take them out of the queue.
The current implementation hangs if tests exceed 2^^15, on Mac OS X.
This might happen with a ninja check-all if one has a bunch of llvm
projects.

Reviewers: delcypher, bkramer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17609

llvm-svn: 263731