Add a test for missing squar-bracket links

[WilcoFiers]

We have on a few occasions missed that links that use square brackets were broken ([hello world][]). We should put tests in place to catch problems like this.

[Jym77]

This is supposed to be done by the link-reference-has-definition test. I agree I've seen it messed up recently. I think it mostly failed on glossary items. Are we somehow only calling that on rules?