Move and simplify element.matches("*") test so that it works (web-platform-tests#11229)

The reason it didn't work before was because in Element-matches.js,
it's always `someElement.matches("*")` being tested, which is the only
thing that could be tested as there's not other "context" or
"reference node" involved in the API surface.

All of TEST_FIND is actually dead code, so dropping that is harmless.

Fixes web-platform-tests#11212.

Author: foolip