[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?
The idea was that we could run this script for every output file (which I didn't implement yet), so by also handing in the filename the script would know which file it needs to process.