Fix buildbot failure in rpl_stop_slave.test.

Problem was setting DEBUG_SYNC twice in a row too fast in the test case; this
could cause the second setting to override the first before the code had time
to react to the first, causing the signal to get lost.

Fixed by waiting for the code to receive the first signal before overwriting
it in the test case.

mysql-test/suite/rpl/r/rpl_stop_slave.result

@@ -168,6 +168,7 @@ STOP SLAVE;
 ROLLBACK;
 [connection master]
 SET DEBUG_SYNC= 'now SIGNAL signal.continue';
+SET DEBUG_SYNC= 'now WAIT_FOR signal.continued';
 SET DEBUG_SYNC= 'RESET';
 [connection slave]
 include/wait_for_slave_to_stop.inc

mysql-test/suite/rpl/t/rpl_stop_slave.test

@@ -109,6 +109,7 @@ ROLLBACK;
 
 --source include/rpl_connection_master.inc
 SET DEBUG_SYNC= 'now SIGNAL signal.continue';
+SET DEBUG_SYNC= 'now WAIT_FOR signal.continued';
 SET DEBUG_SYNC= 'RESET';
 
 --source include/rpl_connection_slave.inc

sql/sql_repl.cc

@@ -571,6 +571,11 @@ impossible position";
                           DBUG_ASSERT(opt_debug_sync_timeout > 0);
                           DBUG_ASSERT(!debug_sync_set_action(current_thd,
                                                              STRING_WITH_LEN(act)));
+                          const char act2[]=
+                            "now "
+                            "signal signal.continued";
+                          DBUG_ASSERT(!debug_sync_set_action(current_thd,
+                                                             STRING_WITH_LEN(act2)));
                         }
                       });