Test for Child::kill after wait

Forgot to submit as part of #7160

tokio/tests/process_kill_after_wait.rs

@@ -0,0 +1,30 @@
+#![warn(rust_2018_idioms)]
+#![cfg(all(feature = "full", not(target_os = "wasi"), not(miri)))] // Wasi cannot run system commands
+
+use tokio::process::Command;
+
+#[tokio::test]
+async fn kill_after_wait() {
+    let mut cmd;
+
+    if cfg!(windows) {
+        cmd = Command::new("cmd");
+        cmd.arg("/c");
+    } else {
+        cmd = Command::new("sh");
+        cmd.arg("-c");
+    }
+
+    let mut child = cmd.arg("exit 2").spawn().unwrap();
+
+    child.start_kill().unwrap();
+
+    child.wait().await.unwrap();
+
+    // Kill after `wait` is fine.
+    child.start_kill().unwrap();
+
+    child.kill().unwrap();
+    child.kill().unwrap();
+    child.kill().unwrap();
+}