Follow-up r92635 Make sure the Exif related tests handle not having the Exif extensio...

Follow-up r92635 Make sure the Exif related tests handle not having the Exif extension available properly

This should fix issues reported in http://lists.wikimedia.org/pipermail/wikitech-l/2011-September/055212.html Lots of these tests loaded the exif extension dynamically, but then didn't reset $wgShowEXIF var properly.

Also change some of the markTestIncomplete to markTestSkipped as that seemed more correct.