How should axe test aria-hidden iframes?

Something like this seems like it should fail `aria-hidden-focus`:

```html

```

What's interesting is that in testing this in VoiceOver, I can actually just get at the button and have it announced correct. I would assume that's because `aria-hidden` doesn't impact content in the frame itself. We should do some testing to see how this behaves.

It might also make sense for `"excludeHidden": false` rules to run inside aria-hidden iframes. Currently I think axe-core just skips these frames. We should do some testing to figure out to what extent screen readers can get at the content inside an aria-hidden (but visible) `iframe`.

Start with the supplied iframe example and compare axe-core's aria-hidden-focus results with keyboard and screen-reader access, including VoiceOver. Investigate whether excludeHidden: false rules run inside visible aria-hidden iframes; no source files or tests are named. Done requires documented behavior and agreement on the expected rule results before an implementation can be scoped.

Written by the indexing model from the issue text.