Move the test format into a standalone .py file and add it to the site
module search path. This allows us to run the test on Windows, and it
makes it compatible with the multiprocessing.Pool lit test execution
strategy.

I think this test was only passing everywhere else because
multiprocessing uses 'fork' to spawn workers, so the test format never
needs to be pickled.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299577 91177308-0d34-0410-b5e6-96231b3b80d8