Eelco has made several early contributions to NixOS including writing
the samba module among other things, but is more or less inactive these
days.
By my brief inspection, he has not committed to the nixos/ tree since
releasing Nix 2.13 in early 2023 and merging a PR to networking tests
slightly before that. A lot of these tests/modules are actually
unmaintained in practice, so we should update the code to reflect the
practical reality so someone can consider picking them up.
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)