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

qobject_to_qstring(obj) returns NULL when obj isn't a QString.  Check
that instead of qobject_type(obj) == QTYPE_QSTRING.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-10-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
