enabling xml validation by default, refs #920

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15419 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
adapting tests to xml validation, refs #920

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@15420 29f149b2-0c63-11e1-8f5c-a347f6a9acd0