Tests: Add unit tests for wp_color_scheme_settings()

[pbearne]

This ticket adds unit tests for the wp_color_scheme_settings() function in wp-admin/includes/misc.php. The tests will verify that the function correctly outputs the color scheme picker and manages the associated scripts and settings for the user profile.

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Description:
This PR adds unit tests for the wp_color_scheme_settings() function in wp-admin/includes/misc.php. These tests ensure that the function correctly outputs the JavaScript configuration for admin color schemes, including appropriate icon color fallbacks when schemes are missing or partially defined.

The tests cover:

• Successful output of icon colors for a valid user-selected color scheme.
• Correct fallback to the "modern" color scheme when an invalid scheme is set in user options.
• Default icon color fallback when neither the current nor the "modern" scheme has defined icon colors.

Trac ticket: https://core.trac.wordpress.org/ticket/65185

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.