Index: tests/phpunit/includes/define-constants.php
===================================================================
--- tests/phpunit/includes/define-constants.php	(revision 8417)
+++ tests/phpunit/includes/define-constants.php	(working copy)
@@ -22,15 +22,15 @@
  */
 if ( false !== getenv( 'WP_TESTS_DIR' ) ) {
 	define( 'WP_TESTS_DIR', getenv( 'WP_TESTS_DIR' ) );
 	define( 'WP_ROOT_DIR', WP_TESTS_DIR );
 } else {
 	// Support WP_DEVELOP_DIR, as used by some plugins
 	if ( false !== getenv( 'WP_DEVELOP_DIR' ) ) {
-		define( 'WP_TESTS_DIR', getenv( 'WP_DEVELOP_DIR' ) );
+		define( 'WP_ROOT_DIR', getenv( 'WP_DEVELOP_DIR' ) );
 	} else {
 		define( 'WP_ROOT_DIR', dirname( dirname( dirname( dirname( dirname( dirname( dirname( __DIR__ ) ) ) ) ) ) ) );
 	}
 
 	define( 'WP_TESTS_DIR', WP_ROOT_DIR . '/tests/phpunit' );
 }