Add I18n-related unit tests (Part 2)

This CR is the first of several dependent CRs that break up the overall
tests added via this abandoned CR:
https://review.openstack.org/#/c/139894
This CR moves the test_repositories.py module to a new 'repository'
package, in anticipation of more repository-related unit tests modules
in future CRs. This CR also refactors the model/repositories.py module
to make it a bit more testable.

Change-Id: I7c6a9b738b86c44031318e74048a1055da822230
Merge "Add I18n-related unit tests (Part 2)"