Problem: rpl_stm_mystery22 is unstable.

Reason: At one place, the test case *should* wait until the SQL thread on the
slave receives an error, but instead it waits until the SQL thread stops. The
SQL thread may stop before the error flag is set, so that when the test case
continues to execute, the error flag is not set.

Fix: Introduce the subroutine mysql-test/include/wait_for_slave_sql_error.inc,
which waits until there is an error in the sql thread of the slave.

Re-commit: fixed one logical error and two smaller things noted by Mats.