Tests: Add unit tests for wp_ajax_find_posts()

[pbearne]

This PR adds unit tests for the wp_ajax_find_posts() function located in src/wp-admin/includes/ajax-actions.php.

The tests cover:

• Successful post searching using the ps POST parameter.
• Proper handling of invalid nonces (find-posts nonce).
• Behavior when no results are found (ensuring it returns a JSON error).

Trac ticket: https://core.trac.wordpress.org/ticket/65252

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