testresults.json: add duration of the tests as well

This is printed by testimage, but isn't actually saved.
It's a useful metric for tracking execution times.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8fc19639)
Signed-off-by: Steve Sakoman <steve@sakoman.com>