Initial xpcshell tests; head, test_helo, and jshintrc

Hi, Kit,

Here is a set of tests for the various outcomes of `HELO` and `AUTH`.

I'm holding off from doing tests for other commands now, because those all assume a successful `AUTH`, and we may want to change the way that's being done in these tests before making a general-purpose wrapper for other methods.

To run the tests, in your `gecko-dev` dir:
    ./mach xpcshell-test dom/channel

Cheers,
j