LP #1677162: Test main.sp-threads is unstable

**Reported in Launchpad by Laurynas Biveinis last update 30-03-2017 08:51:54

On 5.6 trunk:

main.sp-threads w3 [ fail ]
Test ended at 2017-03-28 09:42:18

CURRENT_TEST: main.sp-threads
— /mnt/workspace/percona-server-5.6-trunk/BUILD_TYPE/debug/Host/ubuntu-trusty-32bit/mysql-test/r/sp-threads.result 2017-03-23 15:06:26.843760712 +0300
+++ /mnt/workspace/percona-server-5.6-trunk/BUILD_TYPE/debug/Host/ubuntu-trusty-32bit/build/mysql-test/var/3/log/sp-threads.reject 2017-03-28 16:42:18.595643868 +0300
@@ -34,8 +34,8 @@
call bug9486();
show processlist;
Id User Host db Command Time State Info Rows_sent Rows_examined
+# root localhost test Query # Opening tables update t1, t2 set val= 1 where id1=id2 # #

1. root localhost test Query # STATE show processlist # #
   1. root localhost test Query # Waiting for table metadata lock update t1, t2 set val= 1 where id1=id2 # #
2. root localhost test Sleep # NULL # #
3. root localhost test Sleep # NULL # #
   unlock tables;

This is same issue as https://jira.mariadb.org/browse/MDEV-9970