[TASK] Functional tests without phpunit process isolation

We're finally able to manage our core internal framework state,
at least in the backend. This is a huge step. To proof this,
functional tests now execute without process isolation.

We need a couple of additional reset state methods. Those are
for now marked @internal to allow us changing this stuff if
needed later.

The SystemEnvironmentBuilder also needs a change to not
directly rely on PATH_thisScript anymore.

composer require --dev typo3/testing-framework ~4.1.0

Change-Id: I37fbee7e4cf6ccb2eec18d057b6b9671d6a85167
Resolves: #85649
Releases: master
Reviewed-on: https://review.typo3.org/57129


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>