tests/qapi-schema: Systematic positive doc comment tests

We have a number of negative tests, but we don't have systematic
positive coverage.  Fix that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1490015515-25851-6-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
