Silence potential taskkill error messages

That test is still intermittently failing, because failure to kill the
child process is reported.  Therefore we silence these error messages.

ext/standard/tests/streams/bug78883.phpt

@@ -22,7 +22,7 @@ $bug_is_present = !proc_get_status($proc)['running'];
 if (!$bug_is_present) {
 	// if the bug is not present, it will hang waiting for stdin,
 	// thus cmd is still running and we should kill it
-	shell_exec("taskkill /T /F /PID {$pid}");
+	shell_exec("taskkill /T /F /PID {$pid} 2>nul");
 }
 fclose($pipes[0]);
 fclose($pipes[1]);