Add source-built HorizonTest UI test container

[HanzJas]

Package HorizonTest as openstack-horizontest, matching the tcib image used by test-operator to run Horizon's pytest/selenium UI integration tests against a live dashboard.

run_horizontest.sh and horizontest_sudoers are copied verbatim from tcib; runtime behavior (cloning the Horizon test suite at container start, sudo/crudini/openstack CLI usage) is unchanged.

Differences from tcib's horizontest.yaml, and why:

• Python dependencies (Django, selenium, pytest, horizon, etc.) are resolved via upper-constraints.txt instead of tcib's hardcoded pins, so they track whichever OpenStack release this image targets.
• python-openstackclient is added explicitly. tcib's horizontest image gets the openstack CLI for free because horizontest.yaml is nested under base/os/, inheriting python3-openstackclient from os.yaml. This image has no such inherited layer, so it must be declared.
• junit2html is dropped: it's unused (run_horizontest.sh only relies on pytest's built-in --junitxml and pytest-html's --html), and it ships no sdist on PyPI, which breaks pybuild-deps.
• geckodriver is bumped to v0.37.1 (tcib pins v0.29.0, which only supports Firefox <=90) for compatibility with the current Firefox RPM.

Assisted-By: Cursor Claude Sonnet 5

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign rebtoor for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment