Tests: Make "Traceback did not match" an actual f-string

Tests: Make "Traceback did not match" an actual f-string

Otherwise the failure looks like this:

    >       assert (
                m is not None
            ), "Traceback did not match:\n\n{''.join(tb)}\nexpected:\n{expected_tb}"
    E       AssertionError: Traceback did not match:
    E
    E         {''.join(tb)}
    E         expected:
    E         {expected_tb}
    E       assert None is not None