[Feature Request] Add testing

[phoenixpereira]

Overview

Add pytest as a testing framework and create several tests for each API endpoint.

Details

• Add pytest as a testing framework
• Create unit tests to check querying db, if a basic response returned is 200 for a valid API call, and error handling for invalid requests
• Create about 5 integration tests for each API endpoint with both valid and invalid requests (/subjects, /courses, /courses/{cid})

Why would this feature be useful?

To ensure no regressions.

Additional Notes

Some previous work has been done at #15.
#65 should be fixed first to ensure correct response codes.