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* FileInbound DSL: Add `recursive` for convenience Related to https://stackoverflow.com/questions/66171881/how-to-read-nested-txt-file-from-spring-integration-file The `FileInboundChannelAdapterSpec` can be configured with an external `DirectoryScanner`, but it sometimes becomes burden for end-users to extract a scanner bean and configure it with all the required file filters * Expose a `recursive(boolean)` option for better end-user experience * Rework `FileTests` for JUnit 5 * Mention a new option in the docs * * Restore accidentally removed code * Restore special symbols in the `FileTests` * Fix language in the docs according review