frame-title should flag frames with negative tabindex as incomplete

[WilcoFiers]

Inapplicable example 3 should not fail the frame-title rule:

<iframe tabindex="-1" src="/test-assets/SC4-1-2-frame-doc.html"> </iframe>

Instead of failing these, I think we should report them as incomplete. It largely depends on what you're doing with this element whether or not it needs an accessible name.

[straker]

Work was completed in https://github.com/dequelabs/axe-core/commit/f61f825843ec323fbe7ebb7096bf4c7341f10feb but accidentally pushed to develop directly instead of a pr. The negative tabindex will cause the rule to not run, meaning it will return as inapplicable

[padmavemulapati]

Validated with the latest develop branch code base, and frame-title with negative tabindex in going in inapplicable