Re-enable gemnasium auto-remediation test

What does this MR do?

This MR re-enables the pending gemnasium auto-remediation test by doing the following:

1. Adds a new qa/fixtures/js-yarn/remediate-top-level/git-dir-for-autoremediation git fixture dir.
2. Mounts the above new git fixture dir at $WORKDIR/.git in the Docker container using the mount_points option provided by Add mount_points argument to DockerRunner (integration-test!43 - merged).

What are the relevant issue numbers?

Fix "recorded report" test for auto-remediation... (gitlab-org/gitlab#370486 - closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer