unit_test: Add template for Quick Start

Created by: nicola-mazzucato-arm

It's been some time since we introduced unit testing in the SCP-firmware project. Even though the guidelines are pretty useful to get started, we want to simplify even further the adoption of unit testing for the contributors who may still not familiar with it. Thus, we provide a template that can be used as a first copy-paste example which, along with the instructions, can provide a quick starter into unit testing development.