Issue:
======
Change in join order in multi update statement can produce
different results. main reason bug 16767011 and 18449085.
But these fixes are only made on 5.7 back-porting same
thing on 5.6 is little risky as considering to use temp
table for first table in join order is done before conds
are rearranged and pre-processed in make_join_select.
Where as in 5.7+ such decision happens after
make_join_select. This particular change from 5.7 to 5.6
appears risky hence not back-porting the same.

Solution:
=========
Re-writing the tests in 5.6 so it produces consistent
plan (join orders). This solves sporadic failures.