Refactor LevelWindowManagerTest

Summary:
The LevelWindowManagerTest was refactored according to T28204

That included

• switching to the CppUnit test framework
• removing random value tests
• test every property change

Commit History:

• Remove old tests
• Fix comments and minor style issues
• Make layer check more consistent
  
  The layer check inside the "Update"-function is switched to be in line with the layer check inside the "SetAutoTopMostImage"-function. Additionally both comparisons are made consistent (changed "<" to "<=").

• Add new level window manager test

Test Plan: calling this test class

Reviewers: kislinsk, O1 MITK Reviewer Group I, floca, O3 MITK Reviewer Group II

Reviewed By: O1 MITK Reviewer Group I, floca, O3 MITK Reviewer Group II

Subscribers: floca, kalali, kislinsk

Maniphest Tasks: T28204

Differential Revision: https://phabricator.mitk.org/D461