[IMP] contract: Don't depend on external data

The test as it was, leaves to the demo pricelist the control on the price of
the product, so other modules that modifies this pricelist will make the
test to fail.

This is the minimum change needed for avoiding the problem.

contract/tests/test_contract.py

@@ -25,6 +25,13 @@ class TestContractBase(common.SavepointCase):
         cls.template = cls.env['account.analytic.contract'].create(
             cls.template_vals,
         )
+        # For being sure of the applied price
+        cls.env['product.pricelist.item'].create({
+            'pricelist_id': cls.partner.property_product_pricelist.id,
+            'product_id': cls.product.id,
+            'compute_price': 'formula',
+            'base': 'list_price',
+        })
         cls.contract = cls.env['account.analytic.account'].create({
             'name': 'Test Contract',
             'partner_id': cls.partner.id,