[TASK] Extend untangleFilesArray function test in RequestBuilder

Check for cases where the files array is nested more complex or
keywords (e.g. "error") are used as field names.

Run `curl -F "d0[]=@d12.txt" -F "d0[d1][d2][d3]=@d12.txt"
-F "error=@error_file.txt" localhost:8080` against a custom
php script to generate the testing $_FILES array.

Resolves: #82976
Releases: master, 8.7, 7.6
Change-Id: I51fb9da7c87871b6bb1e8ac4de317973f307dd49
Reviewed-on: https://review.typo3.org/55811


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>