perfschema.stage_mdl_global fails in buildbot on very slow builders

perfschema.stage_mdl_global              w19 [ fail ]

        Test ended at 2017-01-12 14:42:22

CURRENT_TEST: perfschema.stage_mdl_global

--- /home/buildbot/build/mysql-test/suite/perfschema/r/stage_mdl_global.result	2017-01-12 07:57:26.000000000 -0500

+++ /home/buildbot/build/mysql-test/suite/perfschema/r/stage_mdl_global.reject	2017-01-12 14:42:21.804961532 -0500

@@ -22,4 +22,6 @@

 user2	stage/sql/init	STATEMENT

 user2	stage/sql/checking permissions	STATEMENT

 user2	stage/sql/Opening tables	STATEMENT

+user2	stage/sql/Waiting for global read lock	STATEMENT

+user2	stage/sql/Opening tables	STATEMENT

 unlock tables;

mysqltest: Result length mismatch