torturethread: sleep a bit in SubThreadFunc to avoid starvation

testtorture, built by MinGW from msys2, got blocked indefinitely.

test/torturethread.c

@@ -35,7 +35,7 @@ static int SDLCALL
 SubThreadFunc(void *data)
 {
     while (!*(int volatile *)data) {
-        ; /* SDL_Delay(10); */ /* do nothing */
+        SDL_Delay(10);
     }
     return 0;
 }