chore(ci): use 1 namespace for ct install tests

[pmalek]

What this PR does / why we need it:

It seems that using a separate namespace for each test in ct install ... take a lot of time to delete the namespace completely:

Fri, 11 Oct 2024 12:38:46 GMT Deleting release "kong-qpo65402jm"...
Fri, 11 Oct 2024 12:38:46 GMT release "kong-qpo65402jm" uninstalled
Fri, 11 Oct 2024 12:38:46 GMT Deleting namespace "kong-qpo65402jm"...
Fri, 11 Oct 2024 12:38:46 GMT namespace "kong-qpo65402jm" deleted
Fri, 11 Oct 2024 12:39:12 GMT Namespace "kong-qpo65402jm" terminated.

Instead we can use just 1 namespace and share it which greatly decreases the time required for the test suite to run:

• before: ~~14min https://github.com/Kong/charts/actions/runs/11292449703/job/31408471139
• after: ~~7min https://github.com/Kong/charts/actions/runs/11293815050/job/31412833958?pr=1141

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines