Add Pycord based unit tests

Currently none of the cogs have any unit tests because they all depend upon a running bot connected to the actual Discord API. Unit tests should be written once Pycord implements Pycord-Development/pycord#1677 or once a local api hook for testing is written.

Start by reviewing Pycord-Development/pycord#1677 and determining whether Pycord now supports the required testing behavior; otherwise investigate a local API hook. Define how the cogs can be exercised without a running Discord API, then add unit tests covering their behavior and verify they run independently of Discord.

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