Add Disposal Test and fix an issue where the CopyClipboard test was failing

[a-usr]

Fixes #???? - Include a terse summary of the change or which issue is fixed.

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• 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

[a-usr]

please?

[tig]

Lovely! I'll merge as soon as you address my comments.