During the rework done in commit "use and require explicit exports"
with commit id 7ae4a7e78, the test got overlooked. Add an empty
list of checkpoints to the `obs.compile` invocation as to actually
trigger the osbuild invocation.
Reported-By: Thomas Lavocat <tlavocat@redhat.com>
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 two simple tests to check that the osbuild executable fails with the
right exit codes when passed an invalid manifest or checkpoint.
This reuses test.OSBuild, which is extended to raise CalledProcessError
if needed.
