MB-8464: Fix crash in test_checkpoint_persistence

- test_checkpoint_persistence seen to be failing
sometimes only in Linux due to a race condition
- adjusting the checkpoint_flush_timeout before
notifying_IO_completion during notify_checkpoint_
persisted seems to have stopped this error from
occuring.

Change-Id: I6804cb9dc566e9f0d2ed831c694feda19491762a
Reviewed-on: http://review.couchbase.org/27155
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>