tests: Wait for network *online* targets and/or multi-user targets (more) (backport #12315)

[mergify]

Motivation

More prevalent than I thought in #12302

See also https://github.com/NixOS/nix/actions/runs/12872412321/job/35887830320?pr=12310 which is a failed github-flakes test without "Network is Online"

Context

• Similar to tests: Wait for network *online* and multi-user targets #12302

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

---

This is an automatic backport of pull request #12315 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 6f0bdd9 has failed:

On branch mergify/bp/2.24-maintenance/pr-12315
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 6f0bdd9ae.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tests/nixos/git-submodules.nix
	modified:   tests/nixos/github-flakes.nix
	modified:   tests/nixos/nss-preload.nix
	modified:   tests/nixos/s3-binary-cache-store.nix
	modified:   tests/nixos/sourcehut-flakes.nix

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   tests/nixos/nix-docker.nix

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally