START SLAVE UNTIL MASTER ... specifies only SQL thread to stop.
rpl_slave_skip erronously deployed waiting for stop of both threads.

Corrected with deploying the correct macro.
Notice, earlier a similar bug@47749 was fixed in mysql-trunk.

mysql-test/suite/rpl/t/rpl_slave_skip.test:
  changing two waiting calls for offline of both threads into waiting for SQL to stop.