compiletest: Run tests in their own thread to avoid blocking the schedul...

...er

I believe the calls to waitpid are interacting badly with the message passing that goes
on between schedulers and causing us to have very little parallelism in
the test suite. I don't fully understand the sequence of events that causes
the problem here but clearly blocking on waitpid is something that a
well-behaved task should not be doing.

Unfortunately this adds quite a bit of overhead to each test: one thread, two
tasks, three stacks, so there's a tradeoff. The time to execute run-pass on
my 4-core machine goes from ~750s to ~300s.

This should have a pretty good impact on cycle times.

cc @toddaaro