Fix flakey test.

Original pull request: #4824
Closes #4804

Author: christophstrobl

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/ReactiveTransactionIntegrationTests.java

@@ -452,7 +452,7 @@ public Flux<EventLog> saveWithLogs(Person person) {
 			TransactionalOperator transactionalOperator = TransactionalOperator.create(manager,
 					new DefaultTransactionDefinition());
 
-			return Flux.merge(operations.save(new EventLog(new ObjectId(), "beforeConvert")), //
+			return Flux.concat(operations.save(new EventLog(new ObjectId(), "beforeConvert")), //
 					operations.save(new EventLog(new ObjectId(), "afterConvert")), //
 					operations.save(new EventLog(new ObjectId(), "beforeInsert")), //
 					operations.save(person), //