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

[pbearne]

…/misc.php
Description:
This PR adds unit tests for the wp_refresh_heartbeat_nonces() function in wp-admin/includes/misc.php. This function is responsible for adding refreshed REST API and Heartbeat nonces to the response sent back during Heartbeat requests.

The tests cover:

• Successful addition of rest_nonce and heartbeat_nonce to an existing response array.
• Verification that existing data in the response is preserved.
• Verification that if the nonces already exist in the response, they are correctly overwritten with new, valid nonces.

Trac ticket: https://core.trac.wordpress.org/ticket/65199

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