Figure out frame-focusable-content nested iframes

When making the `frame-focusable-content` rule, adding an integration test to test for nested would break the [frame-title integration test](https://app.circleci.com/pipelines/github/dequelabs/axe-core/1749/workflows/0c9e2545-ae29-4382-8418-e27ad37ff7b5/jobs/27496). We thought it was something to do with ordering (maybe something wasn't being cleaned up), but changing the order [didn't fix the problem](https://app.circleci.com/pipelines/github/dequelabs/axe-core/1744/workflows/f7af99f5-6a03-49aa-b585-c0affdf68e07/jobs/27451).

The fix was to [remove the nested iframe test](https://github.com/dequelabs/axe-core/pull/2785/commits/46eb9beea9ffa920271acce2ea43c3c3304e555e). We need to investigate why the nested iframe breaks frame-title.