Use tumbler-thumbnailing test from tests repository group

The tumbler-thumbnailing test has now moved gitlab.apertis.org/tests/tumbler-thumbnailing.git. The Apertis test plan needs to be updated to use those tests in order to test on immutable rootfs setups.

Signed-off-by: Andre Moreira Magalhaes (andrunko) andre.magalhaes@collabora.co.uk