Make poe manage test use pytest

https://gitlab.developers.cam.ac.uk/uis/devops/hr/rewards/rewards-webapp/-/issues/30 was raised after it was noted that poe manage test does not run pytest which, in turn, means that Django Debug Toolbar's detection of pytest via the IS_RUNNING_TESTS setting is faulty.

Update poe manage test to either run tests using pytest or to error with an informative message directing one to a supported way of running tests.