Two more dosep-paralellization fallout fixes

The first issue is about the flaky test rerun logic. This was grouping
tests by subdir and passing them into walk_and_invoke in the incorrect
form. This part can be just deleted as its not needed anymore.

The second problem (which I noticed while investigating the first one)
was that the "-p" switch was not working in multiprocessing mode. This
happened because we were returning None from process_file instead of a
tuple full of empty values for tests that did not match the -p regex.

Both of these would be caught earlier if python was a more strongly
typed language. :/

llvm-svn: 325519