fix LIST test case

As briefly mentioned in asylumfunk/nodeftpd#6, this test case had fallen into an unexecutable state.

This brings the (single) ```LIST``` test case up to date.

Beyond the cleanup, a few more assertions (or rather, ```should```s) were included and the filesystem lookup is now performed asynchronously.

The only change to the library itself is the padding of ```LIST/NLST/STAT``` responses with a space, to clearly mark them as multiline, non-reply code responses.

```package.json``` whitespace changes resulted from my Mocha installation.