Do not rely on shipments order from sale in test scenario

Closes #15009
(grafted from 5996cbb65d20057bd93e5ddfdf97cafe8e4c4529)

--HG--
branch : 8.0

modules/web_shop_shopify/tests/scenario_web_shop_shopify_product_kit.rst

@@ -265,7 +265,7 @@ Make a partial shipment for a single component::
 Ship remaining::
 
     >>> sale.reload()
-    >>> _, _, shipment = sale.shipments
+    >>> shipment, = [s for s in sale.shipments if s.state == 'waiting']
     >>> shipment.click('assign_force')
     >>> shipment.click('pick')
     >>> shipment.click('pack')