Do not, I repeat, NOT make your test framework fail a test run if it writes any text to stderr! No matter how good of on idea you think it is, it's terrible.
If you absolutely, positively have to do that, then print the reason for this failure in your output log. If you can't think of a proper warning message, feel free to copy paste this one:
THIS TEST FAILED BECAUSE IT WROTE TO STDERR AND SOMEONE HERE (OBVIOUSLY NOT ME) THOUGHT MAKING THAT A HARD ERROR WOULD BE A GOOD IDEA!!!!!!!
Sincerely: a person who has lost hours of his life on this sh*t on multiple occasions and can never get it back.