test/crypto: clarify the difference between ownertrust and validity

This is a subtle difference, but the output of notmuch shouldn't ever
change based on ownertrust itself -- notmuch is intended to show valid
User IDs, and to avoid showing invalid User IDs.

It so happens that setting ownertrust of a key to ultimate sets all
associated user IDs to "full" validity, so the test is correct, but
just misnamed.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>