It's not like this test would be an isolated universe. If someones dies there, they stay dead. If you make an enemy during the test, you have likely made an enemy for life, also outside of the test. Mages strong enough to attempt to become first rank mages are the worst kind of enemies one could make. Possibly worse than demons, as the demons don't operate following the same sort of emotions as humans do. Frieren is an elf, so maybe she wouldn't be as bad an enemy.
That's why I'd probably prefer to catch a wild bird, not a bird someone else already caught.