[BUGFIX] Better mocking in FormProtection tests

Some FormProtection tests rely on an instantiated BE_USER object.
The patch takes care of proper mocking in this case and removes
the BE_USER instantiation from unit test bootstrap.

Change-Id: I85005db863bab10ccd61eb910153733b82e2b65e
Resolves: #52819
Releases: 6.2
Reviewed-on: https://review.typo3.org/24750
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn