nixosTests.systemd is quite heavy, it requires a full graphical system,
which is quite a big of a rebuild if the only thing you want to test is
whether dynamic users work.
This is now moved to an `nscd` test, which tests various NSS lookups,
making extra sure that the nscd path is tested, not the fallback path
(by hiding /etc/nsswitch.conf and /etc/hosts for getent).
nixosTests.resolv is removed. It didn't check for reverse lookups,
didn't catch nscd breaking halfway in between, and also had an
ambiguous reverse lookup - 192.0.2.1 could either reverse lookup to
host-ipv4.example.net, or host-dual.example.net.
