[FIX] mail_group: reference new MailCommon class

TestMailCommon was an empty class in test_mail kept for backward compatibility
while MailCommon is in mail. This is needed as we plan to move test_mail in
another addons path in order to isolate test addons.

Task-3263512

Part-of: odoo/odoo#117606

addons/mail_group/tests/common.py

@@ -3,10 +3,10 @@
 
 from odoo import Command, tools
 from odoo.addons.mail.tests.common import mail_new_test_user
-from odoo.addons.test_mail.tests.common import TestMailCommon
+from odoo.addons.mail.tests.common import MailCommon
 
 
-class TestMailListCommon(TestMailCommon):
+class TestMailListCommon(MailCommon):
 
     @classmethod
     def setUpClass(cls):