[TASK] Namespace SystemEnvironmentBuilderTest

* Move BaseSetupTest to SystemEnvironmentBuilderTest and namespace it.
* Introduce an AccessibleProxy for the protected methods instead of eval.
* Merge config_defaultTest to SystemEnvironmentBuilderTest

Change-Id: Ie90857962087e235a7f4a8eb53df120624fd8780
Resolves: #40836
Related: #40095
Releases: 6.0
Reviewed-on: http://review.typo3.org/14527
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn