[JUNIT] Improve readability of FileList[Diff] test cases

Among other things, this splits the tests for FileList from FileListDiff
(while using a common super class for both), and tests the "units" more
directly.

Change-Id: I40ea087af52fc22b627db181964ba385f70c3970
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1558
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>