nixos/tests: fix opensmtpd under network-online dep fix

Changed files
+1
nixos
+1
nixos/tests/opensmtpd.nix
···
testScript = ''
start_all()
+
client.systemctl("start network-online.target")
client.wait_for_unit("network-online.target")
smtp1.wait_for_unit("opensmtpd")
smtp2.wait_for_unit("opensmtpd")