test: add user module unit test

[arsforza]

closes #1603

Describe your changes

Brief description of your changes - not in-depth because the bulk of the description should be in the task on DevOps.

Checklist before requesting a review

• I have performed a self-review of my code
• I have added tests wherever relevant
• I have made sure that all automated checks pass before requesting a review

Notes for the reviewer

@gulfaraz: @jannisvisser and I decided to add a test on generateJWT because:

• other functionalities are already covered by API tests
• the remaining non-tested methods in the service are just database calls and we felt they didn't need a unit test

[jannisvisser]

@arsforza looks good. I can confirm that this is what we agreed on together. But good for @gulfaraz to also review, and to thereby also look at the user module integration tests.

[gulfaraz]

Nice work @arsforza

[jannisvisser]

Merging this in andrea's absence @gulfaraz