Do not rely on invoices order from sale in sale supply test scenario

Closes #15034
(grafted from 76fa6c0dae6901fcf8eb1df8037dbddb09499b8b)

--HG--
branch : 7.0

modules/sale_supply/tests/scenario_sale_supply_shipment_on_invoice.rst

@@ -115,7 +115,7 @@ Not yet a purchase request::
 Pay for remaining products::
 
     >>> sale.reload()
-    >>> _, invoice = sale.invoices
+    >>> invoice, = [i for i in sale.invoices if i != invoice]
     >>> invoice.click('post')
     >>> pay = invoice.click('pay')
     >>> pay.form.payment_method = payment_method

modules/sale_supply_drop_shipment/tests/scenario_sale_supply_drop_shipment_on_invoice.rst

@@ -132,7 +132,7 @@ Not yet a purchase request::
 Pay for remaining products::
 
     >>> sale.reload()
-    >>> _, invoice = sale.invoices
+    >>> invoice, = [i for i in sale.invoices if i != invoice]
     >>> invoice.click('post')
     >>> pay = invoice.click('pay')
     >>> pay.form.payment_method = payment_method