test: describe naming conventions for macro UNIT_TEST

Strict naming conventions have to be followed for Python function
generate_ut_subtest() to collect C unit tests to be executed via
command 'ut'.

Describe the requirements both on the C as well on the Python side.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>