Improve tests, clean up test code, and sort test functions/classes

[LilSpazJoekp]

This improves the test code and leverages more pytest features.

Summary of changes:

• added a reddit fixture that provides a praw.Reddit instance
• added a image_path fixture that provides easy access to test media files
• integration tests will automatically load a cassette for each test with an auto-generated name
• added pytest markers for configuring the recorder at a test class or function level
• time.sleep is automatically mocked
• a check to prevent unused or empty cassettes from being uploaded
• a warning when an integration test made no requests and should be a unit test