BUG: Fix qSlicerExtensionsManagerModelTest.testExtractExtensionArchive

By using QFile static function each time permission are set/get, we
ensure that the permissions of the file are fully re-stated [1]. Indeed,
internally it create a QFile object for each call.

Doing so fix the test ensuring the call to:

  QFile(this->Tmp.absolutePath()).permissions() & QFile::WriteUser)

returns the expected value just after permission has been set.

This is a know Qt issue. See https://bugreports.qt-project.org/browse/QTBUG-7211

[1] http://en.wikipedia.org/wiki/Stat_%28system_call%29

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22564 3bd1e089-480b-0410-8dfb-8563597acbee