Add form params example to unit testing docs

[mtsbarbosa]

Objective

Improve unit test docs by having form params examples

Background

I took half a day to manage how to send form params in :body of response-for
It is pretty simple, as you might know already
But it would be good anyway to have an example so the next beginners don’t take that long finding it out as well

BTW, I signed the Agreement already in another PR I created few weeks ago