[TASK] Cleanup Unit Tests

* Skip the IPV6 Test that always fails on darwin/ OSX OS
* Remove the potentially skipped tests concerning t3lib_diff
  since this class is still in use

Change-Id: Iab26c428ff7cf8a8b447d536d820821ed000a8fd
Resolves: #40879
Releases: 6.0
Reviewed-on: http://review.typo3.org/14564
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel