Resolve "Test the mixin classes more thoroughly"

What does this MR do?

This MR mainly improves test coverage by adding two further test modules, test_base and test_mixins. It also includes minor bug fixes.

Can this MR be accepted?

• Tests added and/or adjusted
• Pipeline passes
• Changelog entry added
• Version number bumped

Anything to double-check?

Nope.

Related issues

Closes #26 (closed)