[IMP] tests: avoid duplicate tests

Linked to odoo/odoo#148660

In the corresponding community pr, a test is not executed twice when
importing a testClass, a common mistake when writing tests.

Upgrade tests were relying on this side effect to run tests. A flag is
added on the class to enable this behavior in this case only.

closes #35

Signed-off-by: Christophe Simonis (chs) <chs@odoo.com>

Author: Xavier-Do

src/testing.py

@@ -95,6 +95,7 @@ class UpgradeCommon(BaseCase):
 
     change_version = (None, None)
     _abstract = True
+    allow_inherited_tests_method = True
 
     @property
     def key(self):