tests: try to fix a few random failures (#35184)

* Change `Swipe` dispose spy on EventHandler
* Modal hide spy on backdrop hide

js/tests/unit/carousel.spec.js

@@ -1268,6 +1268,7 @@ describe('Carousel', () => {
 
       carousel.dispose()
 
+      expect(carousel._swipeHelper).toBeNull()
       expect(removeEventSpy).toHaveBeenCalledWith(carouselEl, Carousel.EVENT_KEY)
       expect(swipeHelperSpy).toHaveBeenCalled()