binman: Add a tutorial on resolving test-coverage bugs

Provide a short description of how tests work, why they are so critical
and how to resolve gaps in Binman's test coverage.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
4 files changed