[IMP] date_range: Import odoo_test_helper inside setUpClass.

When running unit tests on a odoo container that does not
have the odoo_test_helper package installed, the resulting
"ModuleNotFoundError: No module named 'odoo_test_helper'"
kills the entire run, rather than failing only the tests
that require the module.

Author: amh-mw

date_range/tests/test_date_range_search_mixin.py

@@ -2,7 +2,6 @@
 # Copyright 2021 Opener B.V. <stefan@opener.amsterdam>
 # License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl).
 from dateutil.rrule import MONTHLY
-from odoo_test_helper import FakeModelLoader
 
 from odoo.tests.common import TransactionCase
 
@@ -12,6 +11,7 @@ class TestDateRangeearchMixin(TransactionCase):
     def setUpClass(cls):
         super().setUpClass()
         # Load a test model using odoo_test_helper
+        from odoo_test_helper import FakeModelLoader
         cls.loader = FakeModelLoader(cls.env, cls.__module__)
         cls.loader.backup_registry()
         from .models import TestDateRangeSearchMixin