[TASK] Clean up form protection tests

The form protection tests need a refactoring to use the new flash
message handling. Additionally, the tests are adapted to use better
mocking and accessible proxies where needed.

Change-Id: I5d3b008770bc1b74eeff8fd568f01084915cd226
Resolves: #46772
Related: #44593
Releases: 6.1
Reviewed-on: https://review.typo3.org/19430
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn