Change basket quantity error message in tests as well.

tests/functional/test_basket.py

         self.assertEqual(response.status_code, 200)
         self.assertFalse(response.context['formset'].forms[0].is_valid())
         self.assertIn(
-            "can be bought which has been exceeded because you have multiple lines of the same product.",
+            "Available stock is only %s, which has been exceeded because multiple lines contain the same product." % 2,
             str(response.context['formset'].forms[0].errors)
         )

tests/integration/basket/test_forms.py

         )
         self.assertFalse(form.is_valid())
         self.assertIn(
-            "can be bought which has been exceeded because you have multiple lines of the same product.",
+            "Available stock is only %s, which has been exceeded because multiple lines contain the same product." % 2,
             str(form.errors)
         )