Add pytest unit tests

[pederhan]

This PR adds Pytest unit tests which aim to complement the test suite by testing parts of the code that the integrations tests don't cover well (or at all).

Refactored TokenFile

The TokenFile class has been refactored to make it easier to test. This includes making certain methods public, and changing some methods from class methods to instance methods.

Furthermore, a TypeAdapter has been added to validate the contents of the token file. Following the precedent set by the existing methods, failed validation only prints an error instead of raising an exception.

Inline Snapshots

In order to save time on writing tests, the inline-snapshot package has been used to generate snapshots of values we test against. The usage of this should be documented, but the TL;DR is:

When writing a new test

pytest --inline-snapshot=review

One can also skip the diff view and create snapshots without previewing the changes:

pytest --inline-snapshot=create

NOTE: It's recommended to always use --inline-snapshot=review to see a diff of any modifications.

Fixing failing tests

To update a test after changing something that causes it to fail:

pytest --inline-snapshot=fix

# OR to interactively fix:

pytest --inline-snapshot=review

[terjekv]

Great work, thanks!