Add Label and Role tests for `<model>` element.

[cookiecrook]

Add Label and Role tests for <model> element.

• Display of an inline 3D model whatwg/html#10901 (HTML spec)
• New in-line 3D HTML model element w3c/aria#2508 (ARIA specs)
• Initial Model element tests wpt#51667 (WPT general tests)
• Add Label and Role tests for <model> element. #185 (WPT AX tests)
• <model> Element Explainer
• <model> Element Demo