Products.CMFActivity: Unconditionally change user in dummyGroupMethod

Since
  commit f363ac65
  Author: Vincent Pelletier <vincent@nexedi.com>
  Date:   Wed Mar 23 15:55:46 2022 +0900

      Products.CMFActivity.ActivityTool: Store user object in activity.
user_name on newly created activities is always None. As a result,
activities using dummyGroupMethod are executed within the security
context which spawns the group, which is System Processes, instead of
the user which spawned each activity.
Add a unittest for this.