tests: Add missing file to distribution

In f74c9b98 I added tests.xml to the
repository, but not to the distribution tarball.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>