containers: test imperative and ipv4 in small-release (#15015)

Test imperative containers and declarative containers with ipv4. These two
tests are basically the extraction of the containers test from before.

nixos/release-small.nix

···
     inherit (nixos') channel manual iso_minimal dummy;
     tests = {
       inherit (nixos'.tests)
-        containers
+        containers-imperative
+        containers-ipv4
         firewall
         ipv6
         login