Fixes #3836. SetupFakeDriver sometimes causes failure in the unit test.

[BDisp]

Fixes

• Fixes SetupFakeDriver sometimes causes failure in the unit test. #3836

Proposed Changes/Todos

• Application.Screen was causing this issue because _screen wasn't being set to null after running a test using the SetupFakeDriver.

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
• 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

[BDisp]

This error doesn't happens all the time. It's a racing condition when a unit test that use the SetupFakeDriver after it's finished and another unit test that doesn't use it start and will call the superView.SetRelativeLayout (new (10, 10)); when the Application.Screen was already be set to that size before in the first unit test.

[tig]

Well done. Thanks.