On Windows and OS X, where QStandardPaths does not use XDG_DATA_DIRS/
_HOME and shared-mime-info is not installed, the tests that require
additional shared mime info xml files were never run.
Mend that by using QStandardPaths' test mode instead of setting
XDG_DATA_HOME.
Change-Id: I53b75c293c41c4dac63986dcb88972c2b54d5428
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>