Rethrow an exception in the ManualFlowTests.testConcurrentRegistration()

The test fails sometimes, but its exception in the executor is lost.
Rethrow `exceptionHappened` to be reported into logs

spring-integration-core/src/test/java/org/springframework/integration/dsl/manualflow/ManualFlowTests.java

@@ -470,6 +470,7 @@ public class ManualFlowTests {
 				}
 				catch (Exception e) {
 					exceptionHappened.set(true);
+					throw e;
 				}
 
 			});