When an images does not exist just return `False` instead of
raising a RuntimeError. If anything else goes wrong (unknown
output or hash mismatch) keep the RuntimeError as this is an
unexpected exception.
Similar to the `stage_module` fixture for stage tests this adds
a fixture to test sources modules of osbuild.
The code from `stage_module` and `sources_module` is similar and
could be combined but pytest makes it hard to do this without
having a shared root dir. Given that it's just four lines it
seems easier to just life with the tiny bit of code duplication.
