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)

ci.nix

@@ -1,4 +1,5 @@
 {
   smoke = import ./tests/smoke/test.nix;
   pseudofiles = import ./tests/pseudofiles/test.nix;
+  pppoe = import ./tests/pppoe/test.nix;
 }