Clarify groupadmin behavior in test_igroupadmin.py

[MartinFlores751]

Stemming from the discussion here: #7688 (comment)

Add a test to test_igroupadmin.py that shows a groupadmin can:

1. Create a group.
2. Create a regular user.
3. Add the regular user to the group.
4. Show that the groupadmin is unable to manage the group.

Show that, in step 4, the groupadmin cannot remove the user from the group. Possibly check that the groupadmin is unable to add another user to the group.

Other variations may be added to test if needed.

The purpose of adding this test is so that the code (documentation) shows the expected behavior of a groupadmin.