nixos/nats: fix test

(#160908)

nixos/tests/nats.nix

@@ -45,21 +45,19 @@ in import ./make-test-python.nix ({ pkgs, lib, ... }: {
             "{}"
         ).format(" ".join(args))
 
+    def parallel(*fns):
+        from threading import Thread
+        threads = [ Thread(target=fn) for fn in fns ]
+        for t in threads: t.start()
+        for t in threads: t.join()
+
     start_all()
     server.wait_for_unit("nats.service")
 
-    client1.fail("test -f ${file}")
-
-    # Subscribe on topic on client1 and echo messages to file.
-    client1.execute("({} | tee ${file} &)".format(nats_cmd("sub", "--raw", "${topic}")))
-
-    # Give client1 some time to subscribe.
-    client1.execute("sleep 2")
-
-    # Publish message on client2.
-    client2.execute(nats_cmd("pub", "${topic}", "hello"))
-
-    # Check if message has been received.
-    client1.succeed("grep -q hello ${file}")
+    with subtest("pub sub"):
+        parallel(
+            lambda: client1.succeed(nats_cmd("sub", "--count", "1", "${topic}")),
+            lambda: client2.succeed("sleep 2 && {}".format(nats_cmd("pub", "${topic}", "hello"))),
+        )
   '';
 })