Fix tst_QFileSystemWatcher failure on Mac OS X

Fix tst_QFileSystemWatcher failure on Mac OS X

The watchFileAndItsDirectory() test would fail due to atime updates for
the first watched file coming up to 2 seconds after the file was closed.
Observation shows that the atime has a 2 second resolution on Mac OS X
using HFS+, so add an appropriate delay to make sure that the atime
update from the kqueue based file system watcher can dispatch all
updates.

Task-number: QTBUG-22744
Change-Id: Ie79af20d6b4c154021307c8a8f6d336369720337
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>