Tightened the arrow functions test

The arrow test now checks:
* 1 parameter with no brackets
* multiple parameters
* absence of `prototype` and inability to be used with `new` (Traceur fails this check)
* lexical `this` binding
* lexical `arguments` binding (Firefox fails this check)

This last bullet-point gives me pause, though - I think it isn't too useful or helpful to the reader to report Firefox/Traceur failing support when the "core functionality" (which I regard as basic syntax + lexical `this`) is present, but of course it would be quite incorrect to give them a "Yes". So, I feel that after this is accepted, it should be a priority to implement "partial support" test results, differentiating core functionality checks from full support checks.

Note: I accidentally based this branch off #164, so you ought to merge that before considering this.