TestEnv should allow to specify GitLab Shell and Gitaly repos via an env variable

Similarly to what we've done in SeedHelper with GITLAB_GIT_TEST_REPO_URL, the repo URL for GitLab Shell and Gitaly should be overridable via CI secret variables.

This will prevent pipelines from failing on dev when GitLab.com is down (e.g. https://dev.gitlab.org/gitlab/gitlabhq/builds/1107818).