[TASK] Add functional tests for impexp FlexForm fields

This patch add functional tests for exporting and importing
FlexForm fields of types "group" internal_type "file" and
"file_reference" and a field with a soft reference link to a file.

Resolves: #56612
Releases: 6.2
Change-Id: Ic66257dab15ec7a2fde522be9f74170eec346877
Reviewed-on: https://review.typo3.org/28466
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn