tests: Reduce the size of the test derivation link farm (backport #1882)

[mergify]

This PR reduces the number of test derivations, only creating one derivation per plugin instead of one test derivation per test configuration

#1878

---

This is an automatic backport of pull request #1882 done by Mergify.

[mergify]

Cherry-pick of 71126bf has failed:

On branch mergify/bp/nixos-24.05/pr-1882
Your branch is up to date with 'origin/nixos-24.05'.

You are currently cherry-picking commit 71126bfe.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/default.nix
	both modified:   tests/test-derivation.nix

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 50d8652 has failed:

On branch mergify/bp/nixos-24.05/pr-1882
Your branch is ahead of 'origin/nixos-24.05' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 50d86527.
  (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/fetch-tests.nix

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/default.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

[MattSturgeon]

Closing as we'll handle this manually.

The total changes to backport span several PRs and are kinda complex.

Not sure if we'll cherry-pick or just checkout the files and produce a single backport commit.