nixos/tests/acme: wait for server to run before starting the target

This is really an ordering issue in the ACME module itself,
but while we think of how to fix it, this should at least unflake
the tests.

nixos/tests/acme.nix

@@ -747,6 +747,7 @@ in {
           with subtest("Can remove an alias from a domain + cert is updated"):
               test_alias = f"{server}-{domains[0]}-alias.example.test"
               switch_to(webserver, f"{server}_remove_alias")
+              wait_for_server()
               start_and_wait(webserver, f"acme-finished-{test_domain}.target")
               wait_for_server()
               check_connection(client, test_domain)