[TASK] Re-establish TcaMigration tests for flex form

TcaMigration for flex form fields lived in FormEngine
related TcaFlexPrepare, and has been moved to
FlexFormTools with #99756.
The related tests were located within UnitDeprecated
since they trigger_error(, E_USER_DEPRECATED). Those
have been later removed with #101115 during v13
deprecation removals. This is bad, we do need to know
if this recursive implemention works.

* Move remaining FormEngine TcaFlexPrepareTest from
  unit to functional tests for simplified maintenance.
* Use DI for TcaItemsProcessorFunctions - a class
  used as TCA 'itemsProcFunc' for special BE cases.
* Add tests of flex form TcaMigration to functional
  FlexFormToolsTests. This is done using a dummy
  error handler to have them within casual (non
  deprecated) tests, which should prevent accidential
  removal in the future.
* Move TcaItemsProcessorFunctionsTest unit tests to
  a functional test case for simplified maintenance.

Resolves: #104287
Related: #99756
Related: #101115
Releases: main
Change-Id: I8cfcdb8a42f9029d909cdde31f382541e2285096
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85050


Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>