Testing: Add lint rule to forbid <a title>

[aduth]

Related: #19990, #19735

Based on the following resources, it is my understanding that we should never want to include a title attribute on a link.

• https://core.trac.wordpress.org/ticket/24766
• https://html.spec.whatwg.org/multipage/dom.html#the-title-attribute
• https://silktide.com/blog/2013/i-thought-title-text-improved-accessibility-i-was-wrong
• https://developer.paciellogroup.com/blog/2012/01/html5-accessibility-chops-title-attribute-use-and-abuse/
• https://www.24a11y.com/2017/the-trials-and-tribulations-of-the-title-attribute/
• https://a11yproject.com/posts/title-attributes/

Much like how we already use eslint-plugin-jsx-a11y for linting various accessibility issues in JSX usage, we should be able to forbid the use of this attribute on links.

Implementation Notes:

It is unclear if this might be best implemented as:

• An upstream patch to eslint-plugin-jsx-a11y which is then incorporated into this project.
• A project-specific rule, like many of the existing no-restricted-syntax.
• A new rule in @wordpress/eslint-plugin, and whether or not that should be part of its bundled recommended ruleset.

Important note: title attributes can be expected (encouraged even) for some non-link elements, so the rule should be made specific to links.

Recommendation: An upstream patch could serve as useful validation of the idea. Even if rejected and still desirable for our own project, the implementation could be reused for a custom rule.