Add visual diff test for story ads system layer

[jshamble]

✅ Tests

Added visual diff test and css selector override on load complete (story ads system layer, not testing the ad itself).
This helps us ensure future changes are the same visually.

At this moment we cannot test the ad itself, as iframe functionality is flaky in Percy.
For more info and details on how percy handles iframes, please reach out to Daniel Rozenberg.

Fixes #35474

[calebcordry]

Right now the ad is blank, is this intentional or are we not waiting long enough?

[calebcordry]

Description nits:

The idea with the emojis is that they are for the title; see other PRs for example https://github.com/ampproject/amphtml/pulls

Added visual diff test and css selector override on load complete.
This ensures that all of the css properties are the same visually.

This is a bit of a confusing description, it makes it sound like we introduce some sort of css override when really we are just configuring the test. Something like "Add visual diff test for story ads" is more clear and accurate. If you really want to describe what visual diffs do, you could say something like "this helps us ensure future changes are the same visually" but it's not necessarily css properties that are being checked.

Also please update the title to indicate this is not a test for the ad badge specifically.

These may seem trivial but accurate descriptions can really help if you have to try to find an old PR 6 months from now.