Add default: break; to switches on SDL_Event::type

This is just a nice-to-have, as `SDL_Event::type` in an `Uint32`, so there are no
compiler errors about not handling all `SDL_EventType` enum values.

src/test/SDL_test_common.c

@@ -2422,6 +2422,8 @@ int SDLTest_CommonEventMainCallbacks(SDLTest_CommonState *state, const SDL_Event
     }
     case SDL_EVENT_QUIT:
         return 1;
+    default:
+        break;
     }
 
     return 0;  /* keep going */