Add a test that passes a CmsgSpace that is too large

test/sys/test_socket.rs

@@ -359,12 +359,12 @@ fn test_impl_scm_credentials_and_rights<T>(mut space: ::nix::sys::socket::CmsgSp
         let msg = recvmsg(recv, &iov, Some(&mut space), MsgFlags::empty()).unwrap();
         let mut received_cred = None;
 
-        assert_eq!(msg.cmsgs().count(), 2);
+        assert_eq!(msg.cmsgs().count(), 2, "expected 2 cmsgs");
 
         for cmsg in msg.cmsgs() {
             match cmsg {
                 ControlMessage::ScmRights(fds) => {
-                    assert_eq!(received_r, None);
+                    assert_eq!(received_r, None, "already received fd");
                     assert_eq!(fds.len(), 1);
                     received_r = Some(fds[0]);
                 }