nixos/tests: Fix delegated prefix in prefix-delegation test

Prefix/pool validation since kea 2.4.0 is now complaining about
overlapping and not properly aligned prefix lengths.

nixos/tests/systemd-networkd-ipv6-prefix-delegation.nix

···
             interfaces-config.interfaces = [ "eth1" ];
             subnet6 = [ {
               interface = "eth1";
-              subnet = "2001:DB8:F::/36";
+              subnet = "2001:DB8::/32";
               pd-pools = [ {
-                prefix = "2001:DB8:F::";
+                prefix = "2001:DB8:1000::";
                 prefix-len = 36;
                 delegated-len = 48;
               } ];
               pools = [ {
-                pool = "2001:DB8:0000:0000:FFFF::-2001:DB8:0000:0000:FFFF::FFFF";
+                pool = "2001:DB8:0000:0000::-2001:DB8:0FFF:FFFF::FFFF";
               } ];
             } ];