Similar to the previous commit to include a `inputs_service` fixture
this does the same for `source.SourcesService` imports.
Note that we cannot easily share the helpers so we have to life with
a bit of very similar but duplicated code. To fix this we would have
to have a shared confftest.py that pytest can find. Which would mean
that we need to put the tests under a common dir that is reachable
via __init__.py files (which we currently not have because stages,
inputs etc do not have a __init__.py so python does not considers
them modules).
The current `make_container()` helper is a bit silly (which is
entirely my fault). It requires a container tag as input but all
tests end up creating a random number for this input. So instead
just remove the input and return the container_id from the podman
build in the contextmanager and use that.
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.
