Tests: Add unit tests for wp_print_theme_file_tree()

[pbearne]

This ticket adds unit tests for the wp_print_theme_file_tree() function in wp-admin/includes/misc.php. This function is responsible for outputting the hierarchical file list for the theme editor.

…sc.php
Description:
This PR adds unit tests for the wp_print_theme_file_tree() function in wp-admin/includes/misc.php. These tests ensure that the function correctly outputs the hierarchical file list for the theme editor, handling current file highlighting, aria attributes, and nested folder structures.

The tests cover:

• Outputting a single file that is currently being edited (with appropriate classes and notice).
• Outputting a single file that is not the current file.
• Correct rendering of nested directory structures with appropriate labels and nesting levels.

Trac ticket: https://core.trac.wordpress.org/ticket/65178

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.