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13a6b560 - [red-knot] mdtest runner: include stderr for crashing tests (#15644)

[red-knot] mdtest runner: include stderr for crashing tests (#15644) ## Summary Test executables usually write failure messages (including panics) to stdout, but I just managed to make a mdtest crash with ``` thread 'mdtest__unary_not' has overflowed its stack fatal runtime error: stack overflow ``` which is printed to stderr. This test simply appends stderr to stdout (`stderr=subprocess.STDOUT` can not be used with `capture_output`) ## Test Plan Make sure that the error message is now visible in the output of `uv -q run crates/red_knot_python_semantic/mdtest.py`