add rudimentary test harness, turn example-configuration into a test

tests/smoke/configuration.nix

@@ -0,0 +1,57 @@
+{ config, tools, pkgs } :
+let
+  inherit (tools.networking) interface address udhcpc odhcpc;
+  inherit (tools.services) oneshot longrun bundle output;
+in rec {
+  services.loopback =
+    let iface = interface { type = "loopback"; device = "lo";};
+    in bundle {
+      name = "loopback";
+      contents = [
+        (address iface { family = "inet4"; addr ="127.0.0.1";})
+        (address iface { family = "inet6"; addr ="::1";})
+      ];
+    };
+  services.dhcpv4 =
+    let iface = interface { type = "hardware"; device = "eth0"; };
+    in udhcpc iface {};
+
+  services.dhcpv6 =
+    let iface = interface { type = "hardware"; device = "eth0"; };
+    in odhcpc iface { uid = "e7"; };
+
+  services.ntp = longrun {
+    # the simplest approach at the consumer end  is to require the
+    # producer to create a file per output variable.
+    name = "ntp";
+    run = let s = services;
+              r =  "${pkgs.ntp}/bin/ntp $(cat ${output s.dhcpv4 "ntp_servers"}) $(cat ${output s.dhcpv6 "NTP_IP"})";
+          in (builtins.trace r r);
+    # I don't think it's possible to standardise the file names
+    # generally, as different services have different outputs, but it
+    # would be cool if services that provide an interface could use
+    # the same name as each other. e.g. for anything implementing
+    # addressProvider you might expect (output svc "address") or
+    # (output svc "family") to work. Otherwise switching a network link
+    # from static to dhcp might require reviewing all the downstreams
+    # that refer to it.
+    # Also, services should declare the outputs they provide
+    outputs = [];
+    dependencies = [services.dhcpv4];
+  };
+
+  services.defaultroute4 =
+    let s = services;
+    in oneshot {
+      name = "defaultroute4";
+      up = ''
+        ip route add default gw $(cat ${output s.dhcpv4 "address"})
+        echo "1" > /sys/net/ipv4/$(cat ${output s.dhcpv4 "ifname"})
+      '';
+      down = ''
+        ip route del default gw $(cat ${output s.dhcpv4 "address"})
+        echo "0" > /sys/net/ipv4/$(cat ${output s.dhcpv4 "ifname"})
+      '';
+    };
+  systemPackages = [ pkgs.hello ] ;
+}