Fix flakey test by asserting execution order.

See #4838
Original pull request: #4843

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

@@ -467,7 +467,7 @@ public Flux<Void> saveWithErrorLogs(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), //