Tests: Add unit tests for wp_make_plugin_file_tree(

[pbearne]

This ticket adds unit tests for the wp_make_plugin_file_tree() function in wp-admin/includes/misc.php. This function is responsible for converting a flat list of plugin files into a tree structure for use in the plugin editor.

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Description:
This PR adds unit tests for the wp_make_plugin_file_tree() function in wp-admin/includes/misc.php. These tests ensure that the function correctly converts a flat list of plugin file paths into a hierarchical tree structure, which is used for the file list in the plugin editor.

The tests cover:

• Empty input list.
• Flat list of files.
• Nested directory structures.
• Deeply nested file paths.
• Handling of multiple plugins having files with the same relative name.

Trac ticket: https://core.trac.wordpress.org/ticket/65176

AI Disclosure:

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

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.