The testWindowTimespan() is time-sensitive

The `FluxAggregatorMessageHandlerTests.testWindowTimespan()`
relies on the time-based window function of `100` millis.
Looks like a delay of `Thread.sleep(20)` may cause a resources race
condition and window is released only with one item.

* Disable `testWindowTimespan()` since there is no guarantee with what
timing settings it is going to work stably

**Cherry-pick to `5.5.x`**

spring-integration-core/src/test/java/org/springframework/integration/aggregator/FluxAggregatorMessageHandlerTests.java

@@ -25,6 +25,7 @@
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
@@ -157,6 +158,7 @@ void testCustomCombineFunction() {
 	}
 
 	@Test
+	@Disabled("Time-sensitive")
 	void testWindowTimespan() {
 		QueueChannel resultChannel = new QueueChannel();
 		FluxAggregatorMessageHandler fluxAggregatorMessageHandler = new FluxAggregatorMessageHandler();