sporadic failures of main.default_debug

wait for the first connection to reach the debug sync point
before letting the second connection do its stuff

mysql-test/t/default_debug.test

@@ -11,6 +11,8 @@ select a,b,default(b) from t1;
 set debug_sync='after_Item_default_value_calculate WAIT_FOR go';
 send select a,b,default(b) from t1;
 connection default;
+let $wait_condition=select count(*) from information_schema.processlist where s
+source include/wait_condition.inc;
 set debug_sync='ha_write_row_start SIGNAL go';
 insert t1 values (100,default(b));
 connection con1;