RetryingTest for FluxAggregatorMessageHandlerTests

Looks like timing is a bottleneck in async tests.
Making it `@RetryingTest(10)` might give more chances for CPU
to deal with our test execution

(cherry picked from commit 3cc015b)

Author: artembilan

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

@@ -26,6 +26,7 @@
 
 import org.assertj.core.api.InstanceOfAssertFactories;
 import org.junit.jupiter.api.Test;
+import org.junitpioneer.jupiter.RetryingTest;
 import reactor.core.publisher.Flux;
 import reactor.test.StepVerifier;
 
@@ -156,7 +157,7 @@ void testCustomCombineFunction() {
 		fluxAggregatorMessageHandler.stop();
 	}
 
-	@Test
+	@RetryingTest(10)
 	void testWindowTimespan() {
 		QueueChannel resultChannel = new QueueChannel();
 		FluxAggregatorMessageHandler fluxAggregatorMessageHandler = new FluxAggregatorMessageHandler();