Add WPT test for getAttributeType() with qualified attribute name.
See w3c/trusted-types#496 (comment)
Differential Revision: https://phabricator.services.mozilla.com/D226476
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1917783
gecko-commit: aba2bd9d817f21ac8a590235dc6635861aea19eb
gecko-reviewers: smaug