docs: Added details on TSan to testing.rst

docs: Added details on TSan to testing.rst

Adds TSan details to testing.rst.
This includes background and reference details on TSan,
and details on how to build and test with TSan
both with and without docker.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200609200738.445-13-robert.foley@linaro.org>
Message-Id: <20200612190237.30436-16-alex.bennee@linaro.org>