Add comprehensive test coverage for old files cleanup functionality

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/core-update.feature

@@ -384,3 +384,146 @@ Feature: Update WordPress core
       """
       Success:
       """
+
+  @require-php-7.2
+  Scenario: Old files from $_old_files are cleaned up when upgrading
+    Given a WP install
+
+    When I run `wp core download --version=6.8 --force`
+    Then STDOUT should contain:
+      """
+      Success: WordPress downloaded.
+      """
+
+    # Create files that should be removed according to 6.9 old_files list
+    Given a wp-includes/blocks/post-author/editor.css file:
+      """
+      /* Old CSS file */
+      """
+    And a wp-includes/blocks/post-author/editor.min.css file:
+      """
+      /* Old minified CSS */
+      """
+    And a wp-includes/blocks/post-author/editor-rtl.css file:
+      """
+      /* Old RTL CSS */
+      """
+    And a wp-includes/blocks/post-author/editor-rtl.min.css file:
+      """
+      /* Old RTL minified CSS */
+      """
+    And a wp-includes/SimplePie/src/Core.php file:
+      """
+      <?php
+      // Old SimplePie Core file
+      """
+    And a wp-includes/SimplePie/src/Decode directory
+
+    When I run `wp core update --version=6.9 --force`
+    Then STDOUT should contain:
+      """
+      Success: WordPress updated successfully.
+      """
+    And the wp-includes/blocks/post-author/editor.css file should not exist
+    And the wp-includes/blocks/post-author/editor.min.css file should not exist
+    And the wp-includes/blocks/post-author/editor-rtl.css file should not exist
+    And the wp-includes/blocks/post-author/editor-rtl.min.css file should not exist
+    And the wp-includes/SimplePie/src/Core.php file should not exist
+    And the wp-includes/SimplePie/src/Decode directory should not exist
+
+  @require-php-7.2
+  Scenario: Custom old_files list is used for cleanup
+    Given a WP install
+
+    When I run `wp core download --version=6.8 --force`
+    Then STDOUT should contain:
+      """
+      Success: WordPress downloaded.
+      """
+
+    # Create test files that we'll add to custom old_files list
+    Given a wp-includes/test-old-file-1.php file:
+      """
+      <?php
+      // Test old file 1
+      """
+    And a wp-includes/test-old-file-2.php file:
+      """
+      <?php
+      // Test old file 2
+      """
+    And a wp-includes/test-old-dir directory
+    And a wp-includes/test-old-dir/test-file.php file:
+      """
+      <?php
+      // Test file in old directory
+      """
+
+    # Modify update-core.php to include our test files in old_files list
+    And a wp-admin/includes/update-core-custom.php file:
+      """
+      <?php
+      global $_old_files;
+      require_once ABSPATH . 'wp-admin/includes/update-core.php';
+      $_old_files[] = 'wp-includes/test-old-file-1.php';
+      $_old_files[] = 'wp-includes/test-old-file-2.php';
+      $_old_files[] = 'wp-includes/test-old-dir';
+      """
+
+    # Backup original and replace with custom version
+    When I run `mv wp-admin/includes/update-core.php wp-admin/includes/update-core-backup.php`
+    And I run `mv wp-admin/includes/update-core-custom.php wp-admin/includes/update-core.php`
+
+    When I run `wp core update --version=6.9 --force`
+    Then STDOUT should contain:
+      """
+      Success: WordPress updated successfully.
+      """
+
+    # Restore original update-core.php
+    When I run `mv wp-admin/includes/update-core-backup.php wp-admin/includes/update-core.php`
+
+    # Verify custom old files were removed
+    Then the wp-includes/test-old-file-1.php file should not exist
+    And the wp-includes/test-old-file-2.php file should not exist
+    And the wp-includes/test-old-dir directory should not exist
+
+  @require-php-7.2
+  Scenario: Old files cleanup works when checksums unavailable
+    Given a WP install
+
+    When I run `wp core download --version=6.8 --force`
+    Then STDOUT should contain:
+      """
+      Success: WordPress downloaded.
+      """
+
+    # Create files that should be removed
+    Given a wp-includes/test-cleanup-file.php file:
+      """
+      <?php
+      // Test cleanup file
+      """
+
+    # Modify update-core.php to include our test file
+    And a wp-admin/includes/update-core-custom.php file:
+      """
+      <?php
+      global $_old_files;
+      require_once ABSPATH . 'wp-admin/includes/update-core.php';
+      $_old_files[] = 'wp-includes/test-cleanup-file.php';
+      """
+
+    When I run `mv wp-admin/includes/update-core.php wp-admin/includes/update-core-backup.php`
+    And I run `mv wp-admin/includes/update-core-custom.php wp-admin/includes/update-core.php`
+
+    # Update to a version where checksums might not be available
+    When I try `wp core update --version=nightly --force`
+    Then STDOUT should contain:
+      """
+      Cleaning up files...
+      """
+
+    When I run `mv wp-admin/includes/update-core-backup.php wp-admin/includes/update-core.php`
+
+    Then the wp-includes/test-cleanup-file.php file should not exist