Fix sporadic test failure in main.processlist

The test runs a query in one thread, then in another queries the processlist
and expects to find the first thread in the COM_SLEEP state. The problem is
that the thread signals completion to the client before changing to COM_SLEEP
state, so there is a window where the other thread can see the wrong state.

A previous attempt to fix this was ineffective. It set a DEBUG_SYNC to handle
proper waiting, but unfortunately that DEBUG_SYNC point ended up triggering
already at the end of SET DEBUG_SYNC=xxx, so the wait was ineffective.

Fix it properly now (hopefully) by ensuring that we wait for the DEBUG_SYNC
point to trigger at the end of the SELECT SLEEP(), not just at the end of
SET DEBUG_SYNC=xxx.

mysql-test/r/processlist.result

@@ -7,7 +7,9 @@ SELECT ID, TIME,TIME_MS FROM INFORMATION_SCHEMA.PROCESSLIST WHERE CONCAT(":", ID
 SET DEBUG_SYNC = 'now SIGNAL fill_schema_proceed';
 ID	TIME	TIME_MS
 TID	0	0.000
-SET DEBUG_SYNC = 'dispatch_command_end SIGNAL query_done';
+SET DEBUG_SYNC = 'dispatch_command_end SIGNAL query_done EXECUTE 2';
+SET DEBUG_SYNC= 'now WAIT_FOR query_done';
+SET DEBUG_SYNC= 'now SIGNAL nosignal';
 select sleep(5);
 sleep(5)
 0

mysql-test/t/processlist.test

@@ -28,7 +28,15 @@ connection default;
 #
 
 connection con1;
-SET DEBUG_SYNC = 'dispatch_command_end SIGNAL query_done';
+# Trigger a signal once the thread has gone from "Query" to "Sleep" command
+# state. Note we need to execute this twice: Once at the end of SET DEBUG_SYNC,
+# and once for the intended time, at the end of SELECT SLEEP().
+SET DEBUG_SYNC = 'dispatch_command_end SIGNAL query_done EXECUTE 2';
+connection default;
+# Wait for and clear the first signal set during SET DEBUG_SYNC.
+SET DEBUG_SYNC= 'now WAIT_FOR query_done';
+SET DEBUG_SYNC= 'now SIGNAL nosignal';
+connection con1;
 select sleep(5); #run a query that will take some time
 connection default;