We want to extend our base-class to support extensions to
unittest.TestCase, so make sure we inherit from it.
Adjust all callers to no longer inherit from TestCase, since this is now
done automatically by TestBase.
We currently run pylint on all files we find in the checkout. This is
particularly annoying when we have osbuild-stores in the checkout, which
then contain lots of python files in their stored trees.
Change the pylint test to use `git ls-tree` to find all files in the
index and then only run pylint against the subset that we are interested
in.
Introduce a third test-group called `src` alongside `mod` and `run.
This will contain tests that run against the source code of osbuild.
This initial commit introduces `test/src/test_pylint.py` which will run
the python linter against all our sources.
