[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?
If you want to get rid of the argument and just call things via
  ${script} <in >out
then you could just have default.sh be
```
  #!/bin/sh
  cat
```
(no arguments)