Add sadt tests to a new package

Initially, the sadt tests are retrieved with the source code using apt source. For that, they reside in debian/tests subdirectory. In order to run sadt tests on OSTree image, the tests have to be distributed. Thus, a canterbury-tests-additions package is added. The original file are replaced by relative symlinks to maintain compatibility with existing tests.

Signed-off-by: Frédéric Dalleau frederic.dalleau@collabora.com