tests: Don't check qobject_type() before qobject_to_qlist()

qobject_to_qlist(obj) returns NULL when obj isn't a QList.  Check
that instead of qobject_type(obj) == QTYPE_QLIST.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-9-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
