doc: add writing test chapter to testing.md

Add a tutorial on writing a new test, based on writing unit tests for
sensorhandler.hpp - specifically GetSdrReq, as it's a good candidate for
unit testing.

Included is a nonexhaustive list of testing best practices. More best
practices are always welcome.

Tested: Viewed testing.md with Chrome builtin Markdown interpreter

Change-Id: I418af8f972a5a0ff36b786cda6fdf9b6b308e8d5
Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
1 file changed