Add unit tests for wp_refresh_metabox_loader_nonces() in wp-admin/inc…

[pbearne]

…ludes/misc.php
Description:
This PR adds unit tests for the wp_refresh_metabox_loader_nonces() function in wp-admin/includes/misc.php. These tests ensure that the function correctly refreshes nonces used for meta boxes in the block editor during Heartbeat requests, while verifying user permissions and handling invalid post IDs.

The tests cover:

• Handling of missing or empty Heartbeat data.
• Validation of the post_id parameter (zero, string, non-numeric).
• Verification that no nonces are refreshed if the current user cannot edit the post.
• Successful generation and verification of metabox_loader_nonce and _wpnonce when authorized.

Trac ticket: https://core.trac.wordpress.org/ticket/65198

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• Used for: Code analysis, test implementation, and workflow management.

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