Package D added a test that patches lane_cases and hands spawn_lane_parity.py a
placeholder build dir; package L then made the three Magma tiers read through a new
lane_names, so the placeholder reached a real ctest and the unit label went red on the
integrated tree. Patching lane_names to an empty set on every arm makes those tiers
report "nothing to compare" and leaves the assertion about lane_cases intact.