MBug#687320: Fix sporadic test failures in innodb_mysql.test and partition_innodb_semi_consistent.test

Problem is that these tests run with --innodb-lock-wait-timeout=2 in .opt
(and this is necessary as built-in innodb does not allow to change this
dynamically). This cases another part of the test to occasionally time
out an UPDATE, which subsequently caused the test case to timeout due to
waiting for a condition (successful UPDATE) that never occurs.

Fixed by re-trying the update in case of timeout.

Tested by inserting a sleep() in the connection that the UPDATE is waiting
for, and checking that the retry loops a couple of times until the other
connection is done and COMMITs.