Add unit tests for wp_check_locked_posts() in wp-admin/includes/misc.php

[pbearne]

Description:
This PR adds unit tests for the wp_check_locked_posts() function in wp-admin/includes/misc.php. These tests ensure that the function correctly identifies locked posts for the post list screen and returns the appropriate Heartbeat response data.

The tests cover:

• Handling of empty or missing Heartbeat data.
• Detection of locked posts when they are locked by other users.
• Verification that no lock information is returned if the current user cannot edit the post.
• Correct formatting of the response when collaboration (RTC) is enabled.
• Correct inclusion of avatar URLs and user display names when collaboration is disabled.
• Handling of invalid post keys.

Trac ticket: https://core.trac.wordpress.org/ticket/65195

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