Add some meta schema tests

[Julian]

We should have some tests whose schema is a meta schema and whose instances are schemas.

This would constitute essentially a test suite for the meta schemas, but is also likely useful for downstream implementors (like well, me!).

(This is a note to basically move these two tests here, along with some more, after deciding how to represent these in the suite).

[Julian]

https://github.com/Julian/jsonschema/pull/383/files was a PR from a user way back when who PM'ed me offering to add these, but who I didn't finish explaining how to send upstream here, so possibly we can cherry pick those commits onto this repo as a start.

[Julian]

Random likely-doable idea: the meta schema is a finite document, so we can likely at very least compute coverage over it (and make sure each property is covered).

[Julian]

Considering we backed some of these out (in #718 via #716 (comment)) it actually probably makes sense to close these I think.

Thanks for giving it a shot though.