test(integration): delay screenshots slightly to avoid JS animations

We've seen some nondeterministic failures in CI due to animation race conditions. It seems like these animations are JavaScript based rather than CSS, and there's not a way to disable them. This change adds a 300-millisecond delay before taking the screenshot, which seems to be the default duration (https://github.com/mui-org/material-ui/blob/174b64f8a3adffb8df3c13285f4edc2437a8547a/packages/material-ui/src/internal/animate.js#L8).

Delays are never ideal, but I can't see another approach at the moment. It's also unclear whether this is enough for CI to pass, as the failure rate is far lower locally.

Refs #388, d546124, c12bc98, mui/material-ui#16367

integration/src/browser/utils.ts

@@ -26,6 +26,7 @@ export async function screenshot(
   const frame = await getFrame(item);
 
   await Promise.all([
+    frame.waitForTimeout(300),
     hideTwitterTimelines(frame),
     resetCarousels(frame),
     removeAnimations(frame),