Create img-src-srcset-roles.tentative.html

[scottaohara]

expands the tests for the img element to verify if the correct role is being exposed for the img element depending on if it has empty or non-empty src or srcset attributes, along with how the roles are to be computed if the element is given a name via aria-label, alt, aria-labelledby, title

see https://github.com/w3c/aria/pull/2221

[spectranaut]

@cookiecrook does this need to be tentative?