tests/chutney: Merge chutney-launching scripts

Nick Mathewsonrequested to merge
nickm/arti:chutney-without-proxy into main

Formerly, the "launch a chutney network" code was duplicated in "setup" and "arti-bench", since "setup" always launched an arti proxy, while "arti-bench" didn't want that functionality.

Now the "setup" script launches an arti proxy conditionally, depending on whether the "proxy" argument is given.

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