Fix race condition in ObsPropagationChInterTests

The `QueueChannel.receive()` may answer before the `QueueChannel.send()`
observation is closed

spring-integration-core/src/test/java/org/springframework/integration/channel/interceptor/ObservationPropagationChannelInterceptorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 the original author or authors.
+ * Copyright 2022-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -205,7 +205,7 @@ public class ObservationPropagationChannelInterceptorTests {
 	}
 
 	@Test
-	void observationContextPropagatedOverExecutorChannel() {
+	void observationContextPropagatedOverExecutorChannel() throws InterruptedException {
 		BridgeHandler handler = new BridgeHandler();
 		handler.registerObservationRegistry(this.observationRegistry);
 		handler.setBeanName("testBridge");
@@ -228,6 +228,9 @@ public class ObservationPropagationChannelInterceptorTests {
 				.containsEntry("foo", "some foo value")
 				.containsEntry("bar", "some bar value");
 
+		// Give the observation registry a chance to close all the observations
+		Thread.sleep(100);
+
 		TestObservationRegistryAssert.assertThat(this.observationRegistry)
 				.doesNotHaveAnyRemainingCurrentObservation();