panic/fork test: Do not run on emscripten

fork fails there.  The failure message is confusing: so c.status()
returns an Err, the closure panics, and the test thinks the panic was
propagated from inside the child.

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

@@ -37,6 +37,7 @@ fn exitstatus_display_tests() {
 }
 
 #[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
 fn test_command_fork_no_unwind() {
     let got = catch_unwind(|| {
         let mut c = Command::new("echo");