[TASK] Avoid usages of TYPO3_MODE

All code that is using the constant TYPO3_MODE is hard to test,
as this needs to be encapsulated into various places. All testing framework
places run with TYPO3_MODE=FE which makes it impossible to even
consider making parts of the testing framework compatible running a pure
Frontend-based request in the future.

On top, the constant covers up cross-dependency between core dependencies
(whereas $GLOBALS[TSFE]->fe_user is actually a dependency to EXT:frontend).

Another testing-helper in Extbase's EnvironmentService allows to switch
within Extbase to simulate Frontend behaviour.

Resolves: #91521
Releases: master, 10.4
Change-Id: I85a34029e399b40d0780f907480f9059bfdb0edb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64598


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>