Fix output of test arguments

This PR improves the legibility of the test arguments, which could appear to be the test description. Here is how it looked before:

![before](https://cloud.githubusercontent.com/assets/92085/2614807/ca19d886-bbee-11e3-8270-47e99a4882d4.png)

If we replace `util.inspect` by `JSON.stringify`, the output becomes much nicer:

![after](https://cloud.githubusercontent.com/assets/92085/2614808/ca1c7ff0-bbee-11e3-8cf7-af01f2deff1e.png)