Add tests for Suspensey CSS (#47708)
This PR adds a test to confirm that the component will be committed
after the stylesheet is loaded, by delaying the CSS request in
middleware and then check the active style in `useEffect` (technically
it should be `useLayoutEffect`). Since our current timeout for that is
set to 500ms, I also added a test with 1000ms delay to rule out false
negative cases.
