tests/system/tests/test_groupmod.py: add test for groupmod -U with user list

tests/system/tests/test_groupmod.py: add test for groupmod -U with user list

Add comprehensive test for the groupmod -U option when provided with a
list of users to set group membership. This test verifies:

- Setting initial group membership with multiple users
- Proper membership verification in both group and gshadow entries
- Updating group membership by modifying the user list
- Correct handling of membership changes in group databases

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>