Add backend Python tests for the `/user` endpoint which creates a new user on the server

[rlivings39]

#27 added some testing infrastructure and tests for the authentication system added by issue #2 .

This issue is to add tests for the /user endpoint that will be used to save a new user account in the server & database.

To do this, I'd suggest modifying server/tests/test_authentication.py to

1. Add a new test function, say def test_new_user(client: FlaskClient), similar to the other functions in that file.
2. Call client.post in that new function with the right input data to make a request to create a new user.
3. Ensure that the response status code is a success, e.g. 201
4. Ensure that you can then log in as that new user by calling the /login endpoint

To experiment with the /user endpoint, and others

1. Start the Flask server as shown in CONTRIBUTING.md
2. Open http://127.0.0.1:5000 in your browser and click the link there to get to the API test page
3. Play with the APIs you're looking to test!

CONTRIBUTING.md also shows how to run tests in your terminal and VSCode

[RyanSchofield]

I'd be happy to write a test for this, unless someone else wants to for learning purposes. One suggestion I might add is to add a test that expects an error status code when attempting to add duplicate usernames (I think it currently returns 400 in that case, but maybe 409 would be more suitable)