docs: Add documentation about tests and SBIUnit

This patch contains the documentation for SBIUnit. It describes:

- What is SBIUnit
- Simple test writing scenario
- How we can cover static functions
- How we can "mock" structures in order to test the functions which
operate on them
- SBIUnit API Reference

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>