Add workspace tools image test CI

Issue - Migrate gitlab-workspaces-tools image to gitlab... (gitlab-org/workspaces/gitlab-workspaces-tools#12 - closed)

What does this MR do and why?

In previous MR, we have migrated major flow for building workspaces toolings image from gitlab-workspaces-tools.

In this MR, I am focusing on adding the test CI, so when people trying to test version bump, we have the right place to do so.

Also refactor the fetch_assets script to make it cleaner.

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.