[Piglit] [PATCH 1/4] Add ConcurrentTestPool singleton.

[Eoff, Ullysses A]

These patches restore ctrl-c behavior for the GPU tests while executing
the cpu-only tests concurrently.  However, there is still one exception
right now:

the glslparsertests do not get queued into the threadpool until
after some of the GPU tests run.  This is attributed to the alphabetical
sorting of the tests before they are iterated over for doWork execution.
For example, "glean" tests will run one-by-one on the main thread before
"glslparser" tests are even encountered for concurrent queuing.

I will submit a follow-up patch that fixes this.  Most likely it'll just
sort the tests based on the "runConcurrent" flag prior to iterating over
them so that concurrent queuing is handled first.  Hopefully it's as
simple as that :)