Code review comment for ~aluria/charm-openstack-service-checks/+git/charm-openstack-service-checks:bug/1821316-rewrite-03-addtests

3 stacked MPs have been requested:
* [1] bug/1821316-rewrite-01-restruct -> master
Integrates template-python-pytests into charm-openstack-service-checks current structure.

* [2] bug/1821316-rewrite-02-helpers -> bug/1821316-rewrite-01-restruct
Moves help functions outside of the reactive script and into lib/.

* [3] bug/1821316-rewrite-03-addtests -> bug/1821316-rewrite-02-helpers
Adds unit and functional tests, as well as fixes bugs found while creating the tests.

1. https://code.launchpad.net/~aluria/charm-openstack-service-checks/+git/charm-openstack-service-checks/+merge/364971
2. https://code.launchpad.net/~aluria/charm-openstack-service-checks/+git/charm-openstack-service-checks/+merge/364970
3. https://code.launchpad.net/~aluria/charm-openstack-service-checks/+git/charm-openstack-service-checks/+merge/364969