Tests: Add unit tests for wp_refresh_heartbeat_nonces()

[pbearne]

This ticket adds unit tests for the wp_refresh_heartbeat_nonces() function in wp-admin/includes/misc.php. These tests will verify that the function correctly refreshes the Heartbeat nonce and returns it in the response data.

…/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

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[SergeyBiryukov]

In 62356:

Tests: Add unit tests for wp_refresh_heartbeat_nonces().

Follow-up to [44275].

Props pbearne.
Fixes #65199.