Tests: Isolate the debug-mode multisite files test.

Run the `MS_FILES_REQUEST` case in a separate process with global state preservation disabled. The constant cannot be removed during teardown and must not affect later `wp_debug_mode()` calls.

Follow-up to [64267].

Developed in: #14297

Props shailu25.
See #65893.


git-svn-id: https://develop.svn.wordpress.org/trunk@64363 602fd350-edb4-49c9-b593-d223f7449a82

Author: lancewillett

tests/phpunit/tests/load/wpDebugMode.php

@@ -62,9 +62,16 @@ public function tear_down() {
 	/**
 	 * Test: `wp_debug_mode()` should log, but not display, errors for `ms-files.php`.
 	 *
+	 * Runs in a separate process because it defines `MS_FILES_REQUEST`, which cannot be
+	 * undone by teardown and can affect later `wp_debug_mode()` calls.
+	 *
 	 * @ticket 53493
+	 * @ticket 65893
 	 *
 	 * @since 5.9.0
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
 	 */
 	public function test_ms_files_logs_but_doesnt_display_errors() {
 		/*