Simplified and improved the rate limit test case
Simplified and improved the rate limit test case

- Using Mock::Quick to override Mojo::IOLoop->timer with a mock function
- No need to await the function's promise resolution because the timer
  never fires
- No need to mock the logger object, mocking the timer is more reliable
- Removes the need for Test::Mockify in this test file, but I'm keeping
  them in the dep list for now because I expect to need them in other
  tests.

vsTerminus/Net-Discord,vsTerminus/Mojo-Discord