BUG: Fix ExtensionsManager model tests on MacOSX and Windows

By default [1], on macosx and windows, settings format is not Ini.
This commit ensure this is the case.

[1] http://qt-project.org/doc/qt-4.8/qsettings.html#Format-enum

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22043 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix extractExtensionArchive test on MacOSX

Similarly to what has been done on Windows platform in
r20083 [1] this commit also excludes the readonly destination
path test on MacOSX.

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=20083

See also issue #2053

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22044 3bd1e089-480b-0410-8dfb-8563597acbee