Supertest augmented assert

[fnocetti]

This monkey-patches supertest to return useful messages on status code assertion failure.
Things to know:

• There's no api to extend supertest (there is for superagent, but not useful for us)
• Monkey patching is done in a non-exported module from the library. That means that should the implementation change, we may find issues.
• The extension clears the stack trace from the error because it's not relevant and only adds noise. The stack trace comes from the moment the error is instantiated and that's not the place in which the validator runs, but supertest itself.
• Since we import supertest in the tests right away, there's not intermediate module to execute the extension within. That said, the best place I found was the testingRoutes module.

PR checklist:

• Update READ.me ?
• Update API documentation ?

QA checklist:

• Smoke test the functionality described in the issue
• Test for side effects
• UI responsiveness
• Cross browser testing
• Code review