Caching coverage output for use between tests.

[leonardoz15]

I think it would be beneficial to save our captured coverage report form the previous test into to the provided pytest cache. With this information in the cache, we can use pytest's provided cache class to set and retrieve this information between test executions.

Our previous option was to save this information into a text file, but since it would need to be accessed on every test suite execution, this file wouldn't be able to be temporary. The advantage of using the pytest cache is that it saves information between test suite executions, and is already hidden as a convention of pytest, which our users would be using regardless.