Fixes #3592. DateField unit tests fail on macos.latest.

[BDisp]

Fixes

• Fixes DateField unit tests fail on macos.latest #3592

Proposed Changes/Todos

• Add conditional for macos

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

[dodexahedron]

This works, but a more formal/appropriate means of achieving platform-specific tests is to use per-platform test methods that have the [SupportedOSPlatform(string)] attribute applied.

For example: [SupportedOSPlatform("macos")]

There is also an inverse attribute available: `[UnsupportedOSPlatform(string)]'

Both can appear multiple times if multiple platforms need to be specified.

[dodexahedron]

That can be used in other code, too - not just tests, by the way.

[BDisp]

Your suggestion is to only run a unit test that only supports or does not support a certain platform. In this case it is a unit test that runs on all platforms (Windows, Linux and Mac) and there are only constraints in 3 situations on the Mac. I also prefer to use the OSPlatform enum because it is more generic.