Fixes https://github.com/spring-projects/spring-integration/issues/3986
The `ReactiveStreamsConsumer` in addition to plain `Subscriber` and `MessageHandler`
also supports a `ReactiveMessageHandler`, but `MockIntegrationContext` doesn't
handle a scenario when `ReactiveMessageHandler` is provided for consumer
* Rework the logic in the `MockIntegrationContext` to substitute a `ReactiveMessageHandler`
in the `ReactiveStreamsConsumer` which has a precedence in its logic over plain `Subscriber`.
* Wrap a plain `MessageHandler` mock into a `ReactiveMessageHandler` before substitution
in the `ReactiveStreamsConsumer`
* Reset `ReactiveStreamsConsumer.reactiveMessageHandler` with source `ReactiveMessageHandler`
or `null` respectively to an original `ReactiveStreamsConsumer` configuration
* Mention `ReactiveMessageHandler` use-case in the `testing.adoc`
**Cherry-pick to `5.5.x`**
4283a5532a