check-qjson: Test errors from qobject_from_json()

Pass &error_abort with known-good input.  Else pass &err and check
what comes back.  This demonstrates that the parser fails silently for
many errors.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1488317230-26248-15-git-send-email-armbru@redhat.com>
