[TASK] Add another test scenario to ArrayUtility::flatten()

Enrich existing test data providers of ArrayUtility::flatten()
with another scenario to see how it behaves on details.

Change-Id: I3e97689efc2f72e1a9532b12652c98ecaef67556
Resolves: #97794
Releases: main
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74962


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>