The test for browsers is a-element.html. The reusable format is urltestdata.txt, which is not documented in detail, but is documented through urltestparser.js, which parses it. Documentation welcome!
https://github.com/annevk/url/ hosts some other files that might be of interest if you want to create additional tests. Please try to keep urltestdata.txt in sync with that repository.
Similar to a-element.html it would be trivial to add more tests for other objects that expose links (e.g. URL and ). There's also room for enhancement and bits that require independent tests: