tests: Add "C-unwind" to extern functions that can panic (#1354)

With Rust 1.81 `extern "C"` functions are by-default not allowed to unwind and instead
follow the `panic=abort` strategy. As a consequence, the `#[should_panic]` macro no longer
works on `extern "C"` functions that panic. To mitigate one can change the signature from
`extern "C"` to `extern "C-unwind"` to change the strategy.

This commit does this for any unit test that uses `extern "C"` functions in combination with `#[should_panic]`.

See also rust-lang/rust#116088

Author: volsa