Add a test documentation section to the docs

This commit adds a new section the HACKING and REVIEWING docs. It
explains the guidelines around inline documentation when
contributing new tests.

Change-Id: Ib56723d0c748baf41439975102d1363f81085f50
Updated from global requirements

Change-Id: Ie493cb22421af0498575c2c44deafc4e8a9ef56d
Merge "Updated from global requirements"
Merge "Add a test documentation section to the docs"