Add support for WP < 6.4 in the acceptance tests.

Author: johnbillion

tests/_support/MUPlugin/acceptance.php

@@ -12,7 +12,12 @@
 					_deprecated_argument( 'my_function', '2.0.0' );
 					break;
 				case 'class':
-					_deprecated_class( 'My_Class', '2.0.0' );
+					if ( function_exists( '_deprecated_class' ) ) {
+						_deprecated_class( 'My_Class', '2.0.0' );
+					} else {
+						// Fallback for WordPress < 6.4.0
+						_doing_it_wrong( 'My_Class', 'Class is deprecated since version 2.0.0', '2.0.0' );
+					}
 					break;
 				case 'constructor':
 					_deprecated_constructor( 'My_Class', '2.0.0' );