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.

1 files changed, 7 insertions, 4 deletions

diff --git a/README.md b/README.md
index f7d5cbe..d8f0346 100644
--- a/README.md
+++ b/README.md
@@ -701,14 +701,17 @@ outside the target directory changed.  The tests deploy every combination
 of a source type's options, except for patches, which the features test
 covers, to the deploying machine itself, to a remote target
 as `root`, and to a remote target using `sudo`, and check the result after
-deploying and after deploying again.
+deploying and after deploying again.  The scenarios whose results depend on
+the version of rsync also run with rsync 3.4.1 and 3.5.0, which
+`tests/rsync.nix` builds, whatever version nixpkgs has.
 The tests require KVM and nixpkgs with `pkgs.testers.runNixOSTest`
 (23.05 or later), and use `<nixpkgs>` by default.
 
 ```sh
-nix-build tests            # all tests
-nix-build tests -A flake   # the test for one source type
-nix-build tests -A jq      # populate's jq programs for flakes, without VMs
+nix-build tests                 # all tests
+nix-build tests -A flake        # the test for one source type
+nix-build tests -A rsync-3-5-0  # the scenarios depending on rsync's version
+nix-build tests -A jq           # populate's programs for flakes, without VMs
 ```