Disable nose plugin in pytest

[wookie184]

This fixes an issue with pytest running functions called setup in test files when they shouldn't be run.

The current case is also fixed by https://github.com/python-discord/bot/pull/2229/commits/6217513d7bc5a97afc595af1966dfd722db0084c, but this fix should help prevent future cases.

For more information see https://github.com/python-discord/bot/pull/2229#issuecomment-1204436420.