nixosTests.ferm: fix network timeout

The subtests could start before the server has configured it's IP
addresses and therefore timeout.

(cherry picked from commit 2539542)

nixos/tests/ferm.nix

@@ -56,6 +56,7 @@ import ./make-test-python.nix ({ pkgs, ...} : {
       start_all()
 
       client.wait_for_unit("network-online.target")
+      server.wait_for_unit("network-online.target")
       server.wait_for_unit("ferm.service")
       server.wait_for_unit("nginx.service")
       server.wait_until_succeeds("ss -ntl | grep -q 80")