added login model test

[nifedara]

What does this PR do?

Adds backend test case

• I added a new test file for login model test
• I added a test for OSM user creation
• Contributes to resolving this issue: Improve backend test cases for fAIr

Consideration?

• Although some might suggest to not test a model that doesn't have a return statement, I added test for this as it is nice to have
• I ensured to use Black formatter
• I used coverage to see test coverage but didn't add that to the requirement file. So to check coverage, you will have to install coverage yourself

How to test?

• Clone this repo
• Follow the installation guide
• run docker exec -it api bash to run the following commands in the container:
  • coverage run --omit='*/usr/*' manage.py test tests.test_login_model
  • or just coverage run manage.py test tests.test_login_model
  • coverage report --omit='*/usr/*' (to see a report of the test coverage)