65653 functions wp get update php url

[pbearne]

This PR introduces dedicated unit tests for the wp_get_update_php_url() function in wp-includes/functions.php.
The tests verify:

•The default support URL return value.
•The ability to override the URL using the WP_UPDATE_PHP_URL environment variable.
•The ability to override the URL using the wp_update_php_url filter.
•The function's robustness in falling back to the default URL if a filter or environment variable provides an empty or invalid value.

Trac ticket: https://core.trac.wordpress.org/ticket/65653

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