Require all the tests that compile a manifest to either specify
checkpoints or exports. Convert all the tests that were relying
on implicit exports with v1 manifests to use explicit exports.
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.
