test: Mark tst_QFileSystemWatcher::watchFileAndItsDirectory() as XFAIL

This is a flaky test on Windows 8 64-bit, so marking it as
XFAIL if it is expected to fail.

Task-number: QTBUG-30943
Change-Id: Idd276f80b54fcd5cf295a7e1adebcf0020eaa8ca
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>