Add unit tests for update_recently_edited() in wp-admin/includes/misc…

[pbearne]

….php
Description:
This PR adds unit tests for the update_recently_edited() function in wp-admin/includes/misc.php. These tests ensure that the function correctly manages the list of recently edited files in the recently_edited option.

The tests cover:

• Adding a new file to an empty list.
• Adding a new file to an existing list (maintaining order).
• Ensuring a maximum of 5 files are kept in the list.
• Moving an existing file to the top of the list when it's edited again.
• Handling duplicate entries.

Trac ticket: https://core.trac.wordpress.org/ticket/65174

AI Disclosure:

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• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

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