[TASK] Purge FormProtection in some functional tests

FormProtectionFactory has a static instance cache
that potentially leaks state in between tests.
The patch adds a couple of purgeInstances() calls
to avoid this.

Resolves: #98470
Releases: main, 11.5
Change-Id: Ic3e0aa5b6cbd7b7368aab34b74890d3f7ee04c1b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75891


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>