Write tests for labeled <form> elements exposed as role="form", and role fallback where unlabeled

[rahimabdi]

As part of a WebKit bug (https://bugs.webkit.org/show_bug.cgi?id=233907), @cookiecrook @scottaohara discussed the behavior of unlabeled <form> (or role="form") elements which need tests written, i.e.,:

• <article role="form"> - would fall back to "article"
• <div role="form article"> - would also fall back to article due to the lack of an explicit name

Also, where a form element is supplied with an accname (e.g., either via ARIA, or title attribute for native <form>) these should be exposed as role="form" (as a navigable landmark like role="region").

The form role fallback tests could be added here: https://github.com/web-platform-tests/wpt/blob/b4bde5be7d/wai-aria/role/fallback-roles.html, and labelled forms being exposed as such here: https://github.com/web-platform-tests/wpt/blob/b4bde5be7d17944f07d19b4725dddfff6c8c3da0/wai-aria/role/form-roles.html.