Adding unit tests for the Database

[jhimanish]

List of Unit Tests:

• check database file exists (test.db)
• check that the database is empty at start and has no users or events inside
• test to ensure that a new user can be added into the database and also retrieved using the email as a query
• test to ensure that a new event can be added into the database and also retrieved using the event name as a query
• testing if users can be added to event_attendance table when they register for events. This checks the many-to-many relationship between the Users and Events database.

To run these tests on your end, use the following code:

1. cd api
2. python -m pytest utils/unit_tests/database_test

Note: Without changing your directory to api/, the test does not run correctly because it can't resolve imports in the api.py file. I think this could be due to how we've setup our testing infrastructure. We may want to create a separate folder (outside of api/) for our unit tests (similar to the lab5 directory setup).

Note: This branch merges into feat/db_models as I branched off feat/db_models to create this branch. We should first merge #7 into main which will allow this PR to merge into main normally as well.

[coskos-ops]

LGTM

[zhast]

Looks good, although will need to be adapted to Heroku Postgres once we fully migrate.