tests: Test optional features

Specification has a few optional features. This list is from memory so possibly not complete:

hashes & length in METAFILEs

The single-item dictionary in timestamp and the dictionary in snapshot may or may not contain hashes and/or length. We should check that at least both reasonable options are supported (neither hashes nor length included; and both hashes and length included)

Consistent snapshot

According to spec the "consistent snapshot" feature is SHOULD only. I don't think we're very interested in the "non-consistent" version: I think we should only possibly test it with security in mind -- but have no practical examples at the moment

EDIT: I believe all tests currently implemented require consistent snapshot. I think this is fine, in this aspect we'll be more opinionated than the spec itself: consistent_snapshot really makes much more sense. We should still accept an optional test for non-consistent snapshot... but it's not a priority

Start by reviewing the existing conformance tests and the specification sections for hashes and length in METAFILEs. Add coverage for cases with neither fields and with both fields, and consider non-consistent snapshots only if a practical security case is identified; done means the optional-feature behavior is exercised and the tests pass.