Rewrite config mocking

[vkbo]

Summary:

This PR rewrites how the global CONFIG object is handled during testing. It is no longer mocked and replaced by a test config object, but is instead reset before each test is run. This means there is no need to assign the global CONFIG object to a class attribute at the initialisation of a class so it can be overridden in the tests. Since the CONFIG object is created immediately on first import, it is very tricky to mock.

With this change, CONFIG is accessed directly wherever access to the user's config settings are needed. This should also be marginally faster, as the object is no longer stored in a class dictionary, which adds a bit of overhead when accessing it.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed