[WIP] Test py postprocess

I am looking for a solution to make the tests more robust. One example is that clang and gcc report a different number of Stokes iterations for the burstedde benchmark in the screen-output. One idea is to allow a test specific script to postprocess the screen-output. This way we can include arbitrary logic in there (fuzzy checking of results, etc.).

This patch implements this using a <test>.py script and the necessary logic in the test system.

Thoughts?
So "screen-output" is both an argument to the script and is the file we write into? That seems duplicative...

Also, _generate_screenoutput seems like the wrong variable name since it doesn't generate it -- it only postprocesses it...