[red-knot] mdtest doesn't tell you the path of the Markdown file if there's a panic

[AlexWaygood]

If there's a panic in an mdtest, mdtest tells you the name of the test that panicked, but not the path to the file that contains the test:

failures:

---- mdtest__call_callable_instance stdout ----
thread 'mdtest__call_callable_instance' panicked at crates/red_knot_python_semantic/src/types/infer.rs:861:18:
no entry found for key
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace