Fixes #4164. FluentTests sometime causes unit tests failures locally and in CI

[BDisp]

Fixes

• Fixes #4164

Proposed Changes/Todos

• [x] Add a lock into the thread code block
• [x] Using WSL I right click in the IntegrationTests.FuentTests and click at "Run Until Failure" during a long time and it didn't fail

Pull Request checklist:

• [x] I've named my PR in the form of "Fixes #issue. Terse description."
• [x] 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.
• [x] My code follows the Terminal.Gui library design guidelines
• [x] I ran dotnet test before commit
• [ ] I have made corresponding changes to the API documentation (using /// style comments)
• [x] My changes generate no new warnings
• [x] I have checked my code and corrected any poor grammar or misspellings
• [x] I conducted basic QA to assure all features are working