Remove the dependency on unittest for the `OSBuild` class which
used the `unittest` instance only for `assertEqual`, which can
easily also be done via a plain `assert`.
Add a simple noop pipeline, with a noop stage and a noop input,
which all in all does nothing much; but it will validate, load
exectue the pipeline, stage and inputs. So maybe not really
"nothing" in the strictest sense.
Add a new trivial runtime-test which simply runs a no-op pipeline. This
is a fast, trivial test that simply verifies osbuild is properly setup
and accessible.
Remove the explicit no-op test from the CI, now that the test-suite has
it as well.
