Fix tests of QMimeDatabase.

Fix tests of QMimeDatabase.

- Use temporary directories to avoid instabilities
  due to remains of previous failed tests and locked
  directories.
- Replace SRCDIR by QFINDTESTDATA(), reference only
  the freedesktop.org.xml contained in the Qt source
  tree by $$QT.corelib.sources.
- Improve some error messages, test suite instructions
  for Windows.

Change-Id: Idee8e3767ef0a8299df3bdaaac20334164878db0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: David Faure <faure@kde.org>