testmessage: Don't push the event if "Retry" was chosen.

Otherwise, the message box thread goes back to blocking, but the app's main
thread progresses to SDL_WaitThread and won't pump the event queue anymore.

On iOS, this means the messagebox won't pop back up (because it uses
SDL_RunOnMainThread, which needs the event loop to pump), but it's just bad
practice on any platform to stop pumping the event queue for indefinite
lengths of time.

test/testmessage.c

@@ -98,15 +98,16 @@ static int SDLCALL button_messagebox(void *eventNumber)
         }
         SDL_Log("Pressed button: %d, %s", button, button == -1 ? "[closed]" : text);
 
+        if (button == 2) {
+            continue;
+        }
+
         if (eventNumber) {
             SDL_Event event;
             event.type = (Uint32)(intptr_t)eventNumber;
             SDL_PushEvent(&event);
         }
 
-        if (button == 2) {
-            continue;
-        }
         return 0;
     }
 }