rhsm_repository: add an initial unit test (#6758)
So far there are no tests for the rhsm_repository module, which makes it
difficult to do even basic changes. Hence, add an initial version of
unit tests, heavily based on the approach of the redhat_subscription
unit tests.
(cherry picked from commit 08fa05f05f)
Co-authored-by: Pino Toscano <ptoscano@redhat.com>
