Add make target test-network running traffic tests in a Chutney network.

This implements ticket #8530.
Use CHUTNEY_PATH to find Chutney.
Invoke binaries in $PATH rather than by absolute path.

That security measure costs more than it gives.
So, keep your PATH sane and we can run this script on more than
one system.
Merge remote-tracking branch 'linus/bug8530'