Add PHPUnit tests for wp_ajax_update_widget error cases

[pbearne]

This PR adds unit tests for the wp_ajax_update_widget() function in src/wp-admin/includes/ajax-actions.php, which delegates to the Customizer's widget update mechanism. The tests cover validation of the AJAX nonce, handling of missing widget IDs, and rejection of template widget updates.
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Trac ticket: https://core.trac.wordpress.org/ticket/65252
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