panic ui test: Provide comprehensive test for panic after fork

This tests that we can indeed safely panic after fork, both
a raw libc::fork and in a Command pre_exec hook.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Co-authored-by: Mara Bos <m-ou.se@m-ou.se>

library/std/src/sys/unix/process/process_unix/tests.rs

@@ -2,6 +2,9 @@ use crate::os::unix::process::{CommandExt, ExitStatusExt};
 use crate::panic::catch_unwind;
 use crate::process::Command;
 
+// Many of the other aspects of this situation, including heap alloc concurrency
+// safety etc., are tested in src/test/ui/process/process-panic-after-fork.rs
+
 #[test]
 fn exitstatus_display_tests() {
     // In practice this is the same on every Unix.