Add libthornbury test definition

This LAVA test definition executes the libthornbury tests using
autopkgtest from its debian package.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6204