Add unit tests for users and groups

[jonandernovella]

Please describe the feature

As a urdr dev, I want to make sure the groups and users endpoints work as expected. Unit tests can help me making sure this is the case.

Detailed description

• Make sure we have users and groups in the db
• Ensure that the groups and user groups endpoints return data in the right format
• Test the data import script fetches the right data from the redmine API