main.join_cache fails in buildbot on very slow builders

main.join_cache 'xtradb'                 w15 [ fail ]

        Test ended at 2017-01-03 17:17:43

CURRENT_TEST: main.join_cache

--- /home/buildbot/build/mysql-test/r/join_cache.result	2017-01-02 14:29:31.000000000 -0500

+++ /home/buildbot/build/mysql-test/r/join_cache.reject	2017-01-03 17:17:42.082081716 -0500

@@ -5706,7 +5706,7 @@

 id	col1

 select timestampdiff(second, @init_time, now()) <= 1;

 timestampdiff(second, @init_time, now()) <= 1

-1

+0

 set join_cache_level=2;

 set @init_time:=now();

 SELECT t.* 

mysqltest: Result content mismatch