Tests: Add unit tests for admin_color_scheme_picker()

[pbearne]

This ticket adds unit tests for the admin_color_scheme_picker() function in wp-admin/includes/misc.php. The tests will cover the correct sorting of color schemes, the default selection, and the HTML output generation including nonces and radio buttons.

…isc.php
Description:
This PR adds unit tests for the admin_color_scheme_picker() function in wp-admin/includes/misc.php. These tests ensure that the function correctly displays the administration color scheme picker, handles user preferences, and maintains the expected sorting order of color schemes.

The tests cover:

• Default selection of the "modern" color scheme when no user preference is set.
• Correct selection of a custom color scheme when set in user meta.
• Fallback to the "modern" scheme when an invalid color scheme is stored.
• Proper sorting of schemes, ensuring "modern", "fresh", and "light" appear first.
• Verification of the generated HTML structure, including fieldsets, labels, radio buttons, and hidden inputs containing theme data.

Trac ticket: https://core.trac.wordpress.org/ticket/65184

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.