Add test to check for schema definitions in specification text

[tsalo]

A simple test would loop through items in the schema, read the description, and grep for that description in the rest of the specification. This stems from #578 (comment).

This is just a stopgap until the schema is used to render the specification text directly (as in #568).

[tsalo]

We ended up merging #568 before starting to work on this, so now the primary entity definitions come directly from the schema, while in-text entity information is supplementary. That doesn't solve the issue for other schema items with descriptions, but I think it does mean that directly checking for schema definitions in the text won't make sense in many cases.