nixosTests.mailman: migrate to runTest

Part of #386873

nixos/tests/all-tests.nix

···
   mailcatcher = runTest ./mailcatcher.nix;
   mailhog = handleTest ./mailhog.nix { };
   mailpit = runTest ./mailpit.nix;
-  mailman = handleTest ./mailman.nix { };
+  mailman = runTest ./mailman.nix;
   man = handleTest ./man.nix { };
   mariadb-galera = handleTest ./mysql/mariadb-galera.nix { };
   marytts = handleTest ./marytts.nix { };

nixos/tests/mailman.nix

···
-import ./make-test-python.nix {
+{ ... }:
+{
   name = "mailman";
 
   nodes.machine =