[IMP] tests: add registry cleanups in Single and Savepoint tests

In some cases, the registry might be updated during a test step
(creating custom models/fields, for example).  In those cases, there
should be an explicit call to `reset_changes` on the registry to make
sure that the next test class starts with a registry that is consistent
with the database state.

Co-Authored-By: Raphaël Collet <rco@odoo.com>

odoo/tests/common.py

@@ -560,6 +560,7 @@ class SingleTransactionCase(BaseCase):
     def setUpClass(cls):
         super().setUpClass()
         cls.registry = odoo.registry(get_db_name())
+        cls.addClassCleanup(cls.registry.reset_changes)
         cls.addClassCleanup(cls.registry.clear_caches)
 
         cls.cr = cls.registry.cursor()