Finished the tests for the DAV class

All DAV methods not tested are not so useful to test or are impossible to test
outside of an HTTPd environment
Changed the API of the DAV to prevent loading stuff when we declare the class
Added tests for DAV_Element_activelock class
Added tests for the DAV_Element_href class
Added tests for the DAV_Element_response class
Added 'testMode' to the DAV class, so we can test stuff which call DAV::header()
Added tests for DAV_Multistatus