Leave a symlink behind for compatibility.

The test still has a dependency on chaiwala-tests for now, so that it
can be run against older versions of apertis-tests.

While I'm editing dependencies, add python, since the test runs it
explicitly.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D2162