Fix issue where the TestViewsDisposeCorrectly was not doing what it was supposed to do

[a-usr]

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit // test fails now (which is good, as the reason for that is that a certain view doesnt dispose correctly.)
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[a-usr]

While testing this i found out that toplevels create multiple references to their subviews that arent being released upon disposal.
Please see #2965

[a-usr]

TestViewsDisposeCorrectly does now sucessfully check wether or not all views can be disposed. Aditionally @BDisp did a great job at fixing an issue where Toplevel was holding references to its subviews even after they where disposed.