test(ngSanitize): enhance test regarding the `xlink:href` attribute

(Related to https://github.com/angular/angular.js/pull/9751#discussion_r19316243 .)
I left them here to showcase that arbitrary (non-URI) attributes (other that `xlink:href`) are sanitized properly.
It is probably not the best place for it here, but I got kind of lazy...

Feel free to submit another PR :)