Initial version of unit testing documentation

This commit includes the 'User guide' and 'Implementing tests' sections
for helping people building, running and writing unit tests. A more
detailed version of documentation is available under the 'Component
user manuals section'.

Change-Id: I67e93ac805d1f4e7727964f3d95a70436ae34733
Signed-off-by: Imre Kis <imre.kis@arm.com>
23 files changed