Add test suite

[Graeme22]

Now that Tastytrade has implemented certification sessions, it seems that it could be realistic to implement a test suite using pytest for catching/preventing bugs in the future. As the project expands with more functionality and the number of users grows, this will be even more important, so this should be a priority for the future.

[Graeme22]

The framework is set up in #94, so now we just need to actually implement the tests! A certification session can be created with the repo secrets as in tests/test_session.py.