[TASK] Replace prophecy in EXT:core FlexFormToolsTest

Resolves: #98753
Releases: main
Change-Id: Ibd1b655f8d7563165683e4d8307c9445ac9d3398
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76574


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>