This adds the possibility to specify a python script for post-processing stdout.

This also adds some system tests for testing the new feature.

NOTRY=true

Change-Id: I0383afb3e23513629508feeb639ed2dfce56b54a
Reviewed-on: https://chromium-review.googlesource.com/443449Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43257}