tests: check output, test rsync versions, close gaps in what's asserted

Several checks would pass with the feature they cover broken, or cover
it only by chance:

- Phases can state what a succeeding populate prints.  That covers the
  warnings of insecureLinks, the lines pruning prints, the one about a
  replaced symlink, and what rsync logs about each item.
- The insecureLinks scenarios take their expectations from the version of
  rsync, so with an rsync older than 3.5.0 they pass without the check
  ever running.  New tests, rsync-3-4-1 and rsync-3-5-0, run them with
  these versions, which tests/rsync.nix builds from their release
  tarballs, with ACLs and extended attributes, which backups use.
- Pruning has a leftover upload to remove, and a source that stops
  having inputs to root leaves no empty directory of roots.
- A full clone has every commit from the start, so the fetch for a commit
  it lacks never runs.  A git scenario deploys a commit that appears on
  the server after cloning.
- The jq test also runs the Nix function that finds a flake's
  subdirectory on the target, over both shapes of nix flake metadata's
  output.

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