For some reason, hiding files via executing "attrib +h filename"
process didn't work realiably, so changed the file hiding to be done
via Windows native API.
Also changed the test to use QTemporaryDir to simplify temporary
directory handling a bit.
Task-number: QTBUG-24291
Change-Id: I4f02b16e2f9105bcf5e6c5bf136f55434a26e2f4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>