tests/acceptance: Enable AVOCADO_ALLOW_UNTRUSTED_CODE in the gitlab-CI

The tests are running in containers here, so it should be OK to
run with AVOCADO_ALLOW_UNTRUSTED_CODE enabled in this case.

Message-Id: <20201023073351.251332-4-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>