CMFActivity.tests: Fix serialization_tag + group method test.

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Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:54:27

failed ERP5.UnitTest-Master

01:40:21

failed ERP5.UnitTest-Master.Medusa

01:48:01

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:20

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:24