Tests: Add unit tests for wp_refresh_metabox_loader_nonces()

[pbearne]

This ticket adds comprehensive unit tests for the wp_refresh_metabox_loader_nonces() function in wp-admin/includes/misc.php. These tests ensure that the function correctly returns refreshed nonces (meta-box-loader and update-post_{$post_id}) when valid data is provided and the user has edit permissions.

…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

AI Disclosure:

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