Refactored quiet and verbosity tests

[kevinlacaille]

We simplified and clarified the planet --quiet and --verbosity tests as a teaching and learning exercise.

[sgillies]

The key things here @kevinlacaille @mkshah605 @jreiberkyle:

• Asserting in "helper" functions or commands can improve clarity and eliminate the need for Python tricks. It requires catch_exceptions=False for CliRunner.invoke, otherwise the AssertionErrors aren't seen by pytest.
• When patching, we need to patch the module that our unit under test is defined in.
• Avoid shoehorning multiple tests into one function because a) that reduces readability and clarity, and b) we lose the ability to run the tests in random order. Tests that run in a fixed order are a great place for bugs to hide.
• If you see if/else in a test function, that can be a sign that the test should be split up. There should be very little conditional logic within a test function.
• If you see for loops in a test function, that can be a sign to use pytest parametrization instead.