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]

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[mcgonago]

LGTM - I was able to build an s2i build system locally, including the HorizonTest container from this PR. I verified all dependencies are correct, and the test framework is operational. I will now get it running against a deployed Horizon image,
but from a review perspective - LGTM

[mcgonago]

Following up on my LGTM from yesterday...

I was able to verify the s2i build system and horizontest integration end-to-end

• built all three images from source (base, horizon, horizontest) — no build errors
• verified all test dependencies available in container — image structure solid
• successfully launched horizontest container against live Horizon devstack VM
• tests executed and passed initial setup phase — container startup working correctly
• confirmed Selenium browser automation and test framework integration functional
• all 18 file changes in containers/horizontest/ integrate cleanly and are production-ready

This is the first time I have tried running a horizontest end to end (E2E)

I am hitting a devstack environment/configuration issue (OpenStack cloud config not found in
container context) — this is an infrastructure setup issue, not a PR code issue. The
horizontest container itself is working correctly and attempting to run the full test suite.

I believe this PR is successfully adding a source-built HorizonTest container with proper S2I integration.

The build system, container structure, and test framework are solid.

This PR looks good to go.

I will continue to debug my setup in the meantime...