Problem: After START SLAVE, the Slave_IO_Status column of
SHOW SLAVE STATUS goes from No to Yes asynchronously. That
caused sporadic failures on pushbuild in rpl_stm_until since
the test contains SHOW SLAVE STATUS right after START SLAVE.
Fix: Wait until Slave_IO_Status becomes Yes after each
START SLAVE.

mysql-test/include/wait_for_slave_io_to_start.inc:
  Macro that waits until the Slave_IO_Running field of
  SHOW SLAVE STATUS becomes Yes.
mysql-test/suite/rpl/r/rpl_stm_until.result:
  updated result file
mysql-test/suite/rpl/t/rpl_stm_until.test:
   - Added wait_for_slave_io_to_start after each start slave.
   - Removed unused initialization of test variable $VERSION
   - Added comments.