Add a symlink in the python 3 system path to maek apertis_tests_lib
available to the system.

Note that this is a somewhat ugly solution. To solve it properly the
build system needs to be a bit revamped to install the python libraries
(and really binaries) in the right spots in the system.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D2201