Fixed Urchin invocation problem in zsh and dash.  

The - currently - pointless invocations of Urchin itself from the various shells have been removed, pending the amendment to Urchin that will allow true cross-shell tests (see #551).

This fixes #565.

That's true for _options_, but not for _positional arguments_. I think you're mistaking `-f` to be the option for specifying the target directory, but it isn't: `-f` is a _standalone_ option that means 'force running even if the target dir name doesn't contain the word "test"'.

Thus, `test/install_script` in the above is the (one and only) _positional_ argument, and placing options _after_ them is typically _not_ supported, and I would recommend against it.