test: improve artifact acceptance test

What does this MR do?

Improve artifact acceptance test to speed up ci: avoid recreating the page process and always assume artifacts-server-timeout=1. Subtests are also run in parallel

See https://gitlab.com/gitlab-org/gitlab-pages/-/pipelines/359800164/test_report

The difference between artifact tests and other acceptance tests is huge.

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines