test: grab the SIGNAL_MTX lock in test_sigaction (#2381)

Author: SteveLauC

test/sys/test_signal.rs

@@ -283,6 +283,7 @@ fn test_from_and_into_iterator() {
 #[test]
 #[cfg(not(target_os = "redox"))]
 fn test_sigaction() {
+    let _m = crate::SIGNAL_MTX.lock();
     thread::spawn(|| {
         extern "C" fn test_sigaction_handler(_: libc::c_int) {}
         extern "C" fn test_sigaction_action(