[TASK] Refactor functional base test class

Extract setUp() and tearDown() code to an own class
and start documenting the whole sytem.

Resolves: #51144
Releases: 6.2

Change-Id: Id71e9f9111f62ee04e5f8ebcec121e5e01b958b3
Reviewed-on: https://review.typo3.org/23154
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel