convert pppoe test to a derivation

* move shell script to a runCommand
* multicast needs special options to run on loopback
 (nix-build sandbox disables non-local network interfaces)

tests/pppoe/test.nix

@@ -0,0 +1,61 @@
+{
+  liminix
+, nixpkgs
+}:
+let img = (import liminix {
+      device = import "${liminix}/devices/qemu/";
+      liminix-config = ./configuration.nix;
+    }).outputs.default;
+    pkgs = import <nixpkgs> {};
+    ros = pkgs.callPackage ../support/ppp-server {};
+    run-qemu = pkgs.writeShellScriptBin "run-qemu" ''
+      export PATH="${pkgs.lib.makeBinPath [pkgs.qemu]}:$PATH"
+      ${builtins.readFile ../../scripts/run-qemu.sh}
+    '';
+
+in pkgs.runCommand "check" {
+  nativeBuildInputs = with pkgs; [
+    python3Packages.scapy
+    expect
+    jq
+    socat
+    ros.routeros
+    run-qemu
+  ] ;
+} ''
+serverstatedir=$(mktemp -d -t routeros-XXXXXX)
+# python scapy drags in matplotlib which doesn't enjoy running in
+# a sandbox with no $HOME, hence this environment variable
+export MPLCONFIGDIR=$(mktemp -d -t routeros-XXXXXX)
+
+killpid(){
+  if test -e $1 && test -d /proc/`cat $1` ; then
+    pid=$(cat $1)
+    kill $pid
+  fi
+}
+
+cleanup(){
+  killpid $serverstatedir/pid
+  test -n "$MPLCONFIGDIR" && test -d "$MPLCONFIGDIR" && rm -rf "$MPLCONFIGDIR"
+  killpid foo.pid
+}
+trap cleanup EXIT
+
+fatal(){
+  err=$?
+  echo "FAIL: command $(eval echo $BASH_COMMAND) exited with code $err"
+  exit $err
+}
+trap fatal ERR
+
+routeros $serverstatedir
+
+run-qemu --background foo.sock ${img}/vmlinux ${img}/squashfs
+expect ${./getaddress.expect}
+
+set -o pipefail
+response=$(python ${./test-dhcp-service.py})
+echo "$response" | jq -e 'select((.router ==  "192.168.19.1") and (.server_id=="192.168.19.1"))'
+echo $response > $out
+''