Add comment explaining PERFLAB_PLACE_OBJECT_CACHE_DROPIN usage in test setup.

Co-authored-by: Felix Arntz <[email protected]>

Author: b1ink0

plugins/performance-lab/tests/test-load.php

@@ -13,6 +13,10 @@ class Test_Load extends WP_UnitTestCase {
 	public function set_up(): void {
 		parent::set_up();
 
+		/*
+		 * This constant is not set by default in production.
+		 * However, it is needed for all tests that cover placement of the object cache drop-in.
+		 */
 		if ( ! defined( 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN' ) ) {
 			define( 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN', true );
 		}