Tests: Add unit tests for set_screen_options()

[pbearne]

This ticket adds unit tests for the set_screen_options() function in wp-admin/includes/misc.php. The tests will cover saving screen options, handling missing or invalid data, and verifying user authentication and nonce checks.

Description:
This PR adds unit tests for the set_screen_options() function in wp-admin/includes/misc.php. These tests ensure that the function correctly saves screen options (like items per page) for the current user, handles non-standard options via filters, and correctly validates input ranges.

The tests cover:

• Saving standard _per_page options.
• Saving custom options using the set-screen-option filter.
• Handling layout_columns options.
• Validating that values are within the allowed range (1-999).
• Correct handling of missing or invalid $_POST data.
• Ensuring the function correctly performs nonce and referer checks.

Trac ticket: https://core.trac.wordpress.org/ticket/65183

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.