fix sdl*_canvas_twice tests, and enable in sdl2

tests/sdl2_canvas_twice.c

@@ -30,7 +30,5 @@ int main(int argc, char **argv) {
   SDL_RenderCopy(renderer, screenTexture, NULL, NULL);
   SDL_RenderPresent(renderer);
 
-  while(1) { SDL_WaitEvent(NULL); }
-
   return 0;
 }

tests/sdl_canvas_twice.c

@@ -20,7 +20,5 @@ int main(int argc, char **argv) {
 
   SDL_Flip(screen);
 
-  while(1) { SDL_WaitEvent(NULL); }
-
   return 0;
 }

tests/test_browser.py

@@ -2276,7 +2276,7 @@ Module['_main'] = function() {
   def test_sdl2_canvas_palette(self):
     self.btest('sdl2_canvas_palette.c', reference='sdl_canvas_palette.png', args=['-s', 'USE_SDL=2'])
 
-  def zzztest_sdl2_canvas_twice(self):
+  def test_sdl2_canvas_twice(self):
     self.btest('sdl2_canvas_twice.c', reference='sdl_canvas_twice.png', args=['-s', 'USE_SDL=2'])
 
   def zzztest_sdl2_gfx_primitives(self):