core(audits): add handling of 'incomplete' results from axe-core

[jayaddison]

Summary
As of [email protected], the form-field-multiple-labels audit will no longer indicate a strict failure, but instead returns an incomplete result.

As per the axe-core docs, an incomplete result indicates either an error executing a rule or an accessibility result which requires manual investigation.

This changeset handles the former error cases by transforming the incomplete result into a lighthouse audit error result.

Non-error incomplete results are considered failure cases.

Related Issues/PRs
Additional background and context is available in #10056, particularly this comment.

[jayaddison]

Resolves #10056 (comment) (cc @patrickhulce)

Unit tests pass locally, but Travis seems unreliable for PRs at the moment. I've opened an experimental improvement in #10075.

[jayaddison]

@patrickhulce Any chance you could reset the Travis build cache for this PR? Doesn't look like I have permissions to, and I think that's all that's left to get this passing (I was quietly hoping that merging master might invalidate the cache, but no dice so far)

[patrickhulce]

Just kicked a new one off without the cache, we'll see if it does the trick!

[patrickhulce]

LGTM % types

awesome job @jayaddison!

[patrickhulce]

@connorjclark any more thoughts?

[jayaddison]

@patrickhulce @connorjclark One thing I was wondering about - the audit explanation field isn't present for the incomplete results here, which is reflected in the updated a11y test expectations (ref).

I've opened an upstream ticket in axe-core about it (linked in this PR's history) to see if that can be addressed, but was wondering if it's likely to cause any issues/problems in the meantime?

[patrickhulce]

if it's likely to cause any issues/problems in the meantime?

It will be a slight regression in the available data, but I don't expect it to be too large of a problem as long as we get this in to 6.0. There's also not much we can do about it other than not upgrade or wait for axe to fix it, so it shouldn't be a blocker IMO.

[jayaddison]

Small update: failureSummary should be available for incomplete results from axe-core>=3.5 (ref).

[patrickhulce]

@paulirish does this require further discussion or does the current state of a review requested from @connorjclark work?

@connorjclark if you'd prefer deferring to my review that's fine too, just wanted to give you the opportunity since you reviewed earlier.

[jayaddison]

Thanks a ton @patrickhulce @connorjclark !