tests: plug leaks when running with --trackmem

(using SDL_VIDEO_DRIVER=x11/wayland on Linux)

test/checkkeysthreads.c

@@ -303,6 +303,9 @@ int main(int argc, char *argv[])
     }
 #endif
 
+    SDL_DestroyRenderer(renderer);
+    SDL_DestroyWindow(window);
+
     SDL_WaitThread(thread, NULL);
     SDL_Quit();
     SDLTest_CommonDestroyState(state);