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.