stop asserting that child.kill() fails after wait

I've confirmed that this test still fails if we reap the child using
waitpid():

    diff --git a/src/lib.rs b/src/lib.rs
    index ca08e94..2b1479d 100644
    --- a/src/lib.rs
    +++ b/src/lib.rs
    @@ -171,7 +171,12 @@ impl SharedChild {
             // and signal the state condvar.
             let mut state = self.state_lock.lock().unwrap();
             // The child has already exited, so this wait should clean up without blocking.
    -        let final_result = noreap_result.and_then(|_| self.child.lock().unwrap().wait());
    +        let final_result = noreap_result.and_then(|_| unsafe {
    +            let mut status = 0;
    +            let pid = self.child.lock().unwrap().id() as libc::pid_t;
    +            libc::waitpid(pid, &mut status, 0);
    +            Ok(std::os::unix::process::ExitStatusExt::from_raw(status))
    +        });
             *state = if let Ok(exit_status) = final_result {
                 Exited(exit_status)
             } else {

Fails with:

    ---- tests::test_into_inner_after_wait stdout ----
    thread 'tests::test_into_inner_after_wait' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 10, kind: Uncategorized, message: "No child processes" }', src/lib.rs:410:22

src/lib.rs

@@ -400,14 +400,8 @@ mod tests {
         shared_child.kill().unwrap();
         shared_child.wait().unwrap();
         let mut child = shared_child.into_inner();
-        // The child has already been waited on, so kill should be an error.
-        let kill_err = child.kill().unwrap_err();
-        if cfg!(windows) {
-            assert_eq!(std::io::ErrorKind::PermissionDenied, kill_err.kind());
-        } else {
-            assert_eq!(std::io::ErrorKind::InvalidInput, kill_err.kind());
-        }
-        // But wait should succeed.
+        // Wait should succeed. (Note that we also used to test that
+        // child.kill() failed here, but its behavior changed in Rust 1.72.)
         child.wait().unwrap();
     }