Instead of running osbuild as a binary use `python3 -m osbuild`
(just like in `test/test.py:compile()`) so that it will use
osbuild fromgit and can be run from a checkout without the need
for an installed osbuild.
There hasn't ever been an official tool for this and everyone was just using
the output from the test to generate the diff. This commit introduces a proper
tool for generating these files.
