BUG#37717: rpl.rpl_stm_until 'stmt' fails sporadically on pushbuild

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.

mysql-test/include/wait_for_slave_io_to_start.inc

@@ -0,0 +1,19 @@
+# ==== Purpose ====
+#
+# Waits until the IO thread of the current connection has started and
+# connected to the master (i.e., until SHOW SLAVE STATUS returns Yes
+# in the Slave_IO_Running field), or until a timeout is reached.
+#
+# ==== Usage ====
+#
+# source include/wait_for_slave_io_to_start.inc;
+#
+# Parameters to this macro are $slave_timeout and
+# $slave_keep_connection. See wait_for_slave_param.inc for
+# descriptions.
+
+let $slave_param= Slave_IO_Running;
+let $slave_param_value= Yes;
+let $slave_error_message= Failed while waiting for slave IO thread to start;
+source include/wait_for_slave_param.inc;
+let $slave_error_message= ;