Improve discoverability of how to test relations

[dwilding]

The how-to guide for relations has a section called Write unit tests. But if I'm starting at the ops.testing reference, I wouldn't know that.

One solution would be to add a link from Relation to the how-to guide. (That link would need to be a full URL, not an intersphinx link, so that it is usable in IDE help).

More generally, Relation probably isn't the only place to do this. Do we need something obvious at the very beginning of the ops.testing reference, which explains that the various how-to guides explain how to test different features?