Tests: Add unit tests for wp_make_theme_file_tree()

[pbearne]

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

…c.php

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

The tests cover:

• Empty input list.
• Flat list of files (no directories).
• Nested list with single-level directories.
• Deeply nested directories.
• Mixed nesting levels.

Trac ticket: https://core.trac.wordpress.org/ticket/65175

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.

[SergeyBiryukov]

In 62388:

Tests: Add unit tests for wp_make_theme_file_tree().

Follow-up to [41851].

Props pbearne.
Fixes #65175.