Fix connection race condition in MQTT test

It looks like `isConnected` flag in the MQTT Client is reset when we already
have subscribed

Author: artembilan

spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/ResubscribeAfterAutomaticReconnectTests.java

@@ -40,6 +40,7 @@
 import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
 
 /**
  * @author Artem Bilan
@@ -81,7 +82,7 @@ void messageReceivedAfterResubscriptionOnLostConnection() throws InterruptedExce
 
 		assertThat(this.config.subscribeSecondLatch.await(10, TimeUnit.SECONDS)).isTrue();
 
-		this.mqttOutFlowInput.send(testMessage);
+		await().untilAsserted(() -> this.mqttOutFlowInput.send(testMessage));
 		assertThat(this.fromMqttChannel.receive(10_000)).isNotNull();
 
 		// Re-subscription on channel adapter restart with cleanStart