testplatform: fix leak at exit time

A leak was present at exit time on the Emscripten platform

test/testplatform.c

@@ -483,6 +483,7 @@ int main(int argc, char *argv[])
     status += Test64Bit(verbose);
     status += TestCPUInfo(verbose);
     status += TestAssertions(verbose);
+    SDL_Quit();
 
     SDLTest_CommonDestroyState(state);