Expose MockState in a gui.test package

[FabienLelaquais]

📌 Description

As of today, the class MockState is defined in taipy/gui/mock/mock_state.py and not exported by any package.

I propose we create the package taipy.gui.test what would export it.

📜 Code of Conduct

• I have checked the existing issues to avoid duplicates.
• I am willing to work on this issue (optional)

✅ Acceptance Criteria

• The refactored code maintains existing functionality without breaking changes.
• Any new code is covered by unit tests.
• Code coverage remains at least 90%.
• Performance improvements are documented, if applicable.

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