Fix ZMQMessageHandlerTests race condition

Use `receiveTimeOut = 0` on SUB socket and rely
on the retries from the `await().atMost()`.
Also resubscribe in the retry callback before publishing the next attempt

spring-integration-zeromq/src/test/java/org/springframework/integration/zeromq/outbound/ZeroMqMessageHandlerTests.java

@@ -86,10 +86,11 @@ public class ZeroMqMessageHandlerTests {
 	}
 
 	@Test
-	void testMessageHandlerForPubSub() throws InterruptedException {
+	void testMessageHandlerForPubSub() {
 		ZMQ.Socket subSocket = CONTEXT.createSocket(SocketType.SUB);
-		subSocket.setReceiveTimeOut(20_000);
+		subSocket.setReceiveTimeOut(0);
 		int port = subSocket.bindToRandomPort("tcp://*");
+		subSocket.subscribe("test");
 
 		ZeroMqMessageHandler messageHandler =
 				new ZeroMqMessageHandler(CONTEXT, "tcp://localhost:" + port, SocketType.PUB);
@@ -98,15 +99,12 @@ public class ZeroMqMessageHandlerTests {
 				new FunctionExpression<Message<?>>((message) -> message.getHeaders().get("topic")));
 		messageHandler.setMessageMapper(new EmbeddedJsonHeadersMessageMapper());
 		messageHandler.afterPropertiesSet();
-		subSocket.subscribe("test");
-
-		// Give it some time to connect and subscribe
-		Thread.sleep(2000);
 
 		Message<?> testMessage = MessageBuilder.withPayload("test").setHeader("topic", "testTopic").build();
 
 		await().atMost(Duration.ofSeconds(20)).pollDelay(Duration.ofMillis(100))
 				.untilAsserted(() -> {
+					subSocket.subscribe("test");
 					messageHandler.handleMessage(testMessage).subscribe();
 					ZMsg msg = ZMsg.recvMsg(subSocket);
 					assertThat(msg).isNotNull();