[lit] Implement timeouts and max_time for process pool testing

This is necessary to pass the lit test suite at llvm/utils/lit/tests.

There are some pre-existing failures here, but now switching to pools
doesn't regress any tests.

I had to change test-data/lit.cfg to import DummyConfig from a module to
fix pickling problems, but I think it'll be OK if we require test
formats to be written in real .py modules outside lit.cfg files.

I also discovered that in some circumstances AsyncResult.wait() will not
raise KeyboardInterrupt in a timely manner, but you can pass a non-zero
timeout to work around this. This makes threading.Condition.wait use a
polling loop that runs through the interpreter, so it's capable of
asynchronously raising KeyboardInterrupt.

llvm-svn: 299605