Use the repo from artifacts in frontend jobs

What does this MR do and why?

The goal is to reduce the load on Gitaly servers by cloning (ultimately) only once per pipeline.

This also reduces the duration of jobs:

• 39 seconds for the clone

  Getting source from Git repository
  00:39
  Fetching changes with git depth set to 20...
  Initialized empty Git repository in /builds/gitlab-org/gitlab/.git/
  Created fresh repository.
  remote: Enumerating objects: 203666, done.        
  remote: Counting objects: 100% (203666/203666), done.        
  remote: Compressing objects: 100% (132220/132220), done.        
  remote: Total 203666 (delta 105143), reused 136129 (delta 63474), pack-reused 0 (from 0)        
  Receiving objects: 100% (203666/203666), 150.24 MiB | 26.86 MiB/s, done.
  Resolving deltas: 100% (105143/105143), done.
  From https://us-east1-c.ci-gateway.int.gprd.gitlab.net:8989/gitlab-org/gitlab
   * [new ref]             refs/pipelines/1358792412 -> refs/pipelines/1358792412
  Checking out e6966359 as detached HEAD (ref is refs/merge-requests/158197/merge)...
  Skipping Git submodules setup
  $ git remote set-url origin "${CI_REPOSITORY_URL}"

• 23 seconds for the artifacts download

  Downloading artifacts
  00:23
  Downloading artifacts for clone-gitlab-repo (7254195929)...
  Downloading artifacts from coordinator... ok        host=storage.googleapis.com id=7254195929 responseStatus=200 OK token=glcbt-66
  WARNING: Part of .git directory is on the list of files to extract 
  WARNING: This may introduce unexpected problems   
  ## MR acceptance checklist

I also introduced a new test-frontend stage to separate frontend tests from backend ones, and moved a few jobs to more relevant stages as well.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.