Fixes: #9561
PR: https://github.com/spring-projects/spring-integration/pull/9561
The `DelayHandler.DelayedMessageWrapper` cannot be deserialized when using `RedisMessageStore` and JSON serialization:
```
org.springframework.data.redis.serializer.SerializationException: Could not read JSON:Cannot construct instance of `org.springframework.integration.handler.DelayHandler$DelayedMessageWrapper` (no Creators, like default constructor, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
at [Source: UNKNOWN; byte offset: #UNKNOWN]
at org.springframework.data.redis.serializer.GenericJackson2JsonRedisSerializer.deserialize(GenericJackson2JsonRedisSerializer.java:311)
```
* More code clean up and refactoring in the test
**Auto-cherry-pick to `6.3.x` & `6.2.x`**
73bb813b53