using randomness only if needed, refs #12

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17368 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
adapting to new randomness again, allowing gui internal tests, refs #21

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17369 29f149b2-0c63-11e1-8f5c-a347f6a9acd0