pytest: measure branch coverage

[adamjstewart]

By default, coverage only reports which lines were executed during unit testing. By enabling branch coverage, we can also see whether if-statements are tested for both True and False conditions.

Reference: https://coverage.readthedocs.io/en/latest/branch.html

This is a WIP until we get 100% coverage. If anyone would be interested in contributing to this, it would be a great first issue.