More refinements to the panic messages for failing mdtests to mimic the
output of the default panic hook more closely:
- We now print out `Box<dyn Any>` if the panic payload is not a string
(which is typically the case for salsa panics).
- We now include the panic's backtrace if you set the `RUST_BACKTRACE`
environment variable.
This fixes#15317. Our `catch_unwind` wrapper installs a panic hook that
captures (the rendered contents of) the panic info when a panic occurs.
Since the intent is that the caller will render the panic info in some
custom way, the hook silences the default stderr panic output.
However, the panic hook is a global resource, so if any one thread was
in the middle of a `catch_unwind` call, we would silence the default
panic output for _all_ threads.
The solution is to also keep a thread local that indicates whether the
current thread is in the middle of our `catch_unwind`, and to fall back
on the default panic hook if not.
## Test Plan
Artificially added an mdtest parse error, ran tests via `cargo test -p
red_knot_python_semantic` to run a large number of tests in parallel.
Before this patch, the panic message was swallowed as reported in
#15317. After, the panic message was shown.
This updates the mdtest harness to catch any panics that occur during
type checking, and to display the panic message as an mdtest failure.
(We don't know which specific line causes the failure, so we attribute
panics to the first line of the test case.)
