ACT: Add fail reason to img_alt_null for conflicts in attributes and map to ACT rule 46ca7f

[philljenkins]

In the ACT test case below the ARIA label does go into the accessibility tree in Chrome and FF, so the accessible name is not empty. However, the developer may be confused because the alt="" was added in their source code, perhaps thinking it would mark the img as decorative, but it wasn't due to the alt attribute being ignored/overridden by the aria attribute because of the accessible name calculation spec for browsers.

Failed Example 2 Open in a new tab
This img element is marked as decorative through its empty alt attribute but has a non-empty aria-labelledby attribute causing it to be included in the accessibility tree with its implicit role of img.

<img src="/test-assets/shared/w3c-logo.png" alt="" aria-labelledby="label" /> 
<span hidden id="label">W3C logo</span>

However, the Checker should be a new warning or recommendation notifying the developer of the contradiction (this issue). The Checker should add a new fail reason code in the image_alt_null (rule source code).

There is another issue against ACT to clarify/ask why the Failed 2 above is a valid fail test case for ACT ruleID 46ca7f: Element marked as decorative is not exposed - see w3c/wcag-act-rules#295