Fix Ubuntu tests for dconf_gnome_login_banner_text

[mpurg]

Description:

• Refactor and fix tests for dconf_gnome_login_banner_text on Ubuntu

Rationale:

• Tests were not aligned with the recent change in banner vars: Store plain login banner text in XCCDF Value #14371

[dodys]

@mpurg tests are failing

[mpurg]

Reverted back to packages = gdm3 which is needed in containers.
Was testing locally with a desktop VM which had the package already installed.

python3 "tests/automatus.py" rule         --libvirt qemu:///system "sec-noble-amd64"         --datastream "build/ssg-ubuntu2404-ds.xml"         --remove-fips-certified         --remediate-using "bash"         --profile "(all)"         --dontclean         "dconf_gnome_login_banner_text" --debug

INFO - xccdf_org.ssgproject.content_rule_dconf_gnome_login_banner_text
INFO - Script ubuntu_wrong_value.fail.sh using profile (all) OK
INFO - Script ubuntu_missing_value.fail.sh using profile (all) OK
INFO - Script ubuntu_correct_value_defaults.pass.sh using profile (all) OK
INFO - Script ubuntu_wrong_value_defaults.fail.sh using profile (all) OK
INFO - Script ubuntu_correct_value.pass.sh using profile (all) OK