[TASK] FunctionalTestCase refactoring

* Make properties in utility class non static, store private instance
* Improve documentation
* Better exception code in case test database can not be created

Change-Id: Ib05ad804fd4bbb6772243343e2160df5170d8bfe
Resolves: #51156
Releases: 6.2
Reviewed-on: https://review.typo3.org/23181
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn