Set up unit testing

[GabrielMajeri]

We should look into writing a few unit tests to ensure the correctness and quality of our code. We can use Jest + ts-jest as a test runner and React Testing Library for interacting with the components being tested.

Possible tests

• The root app component renders correctly (smoke test)
• The payment forms can be filled-in and submitted (we can mock the database for these tests, we only want to check that the UI and the end points work)
• The admin dashboard pages display an error message if the user is not logged in (we can mock the auth state)
• The user can log into the admin page (we can mock the auth flow)