Add rel tests with empty href values

So when I was working on fixing some tests, namely one that mirrored https://webmention.rocks/test/15, I noticed that the Rust mf2 parser couldn't handle empty rels. I've patched it to do so, but running the test suite didn't seem to catch that (at least not in the rel folder for microformats2).

(Originally published at: https://jacky.wtf/2022/6/n_Or)