Car and Driver tested a $1,000 aftermarket autonomous driving add-on from Comma.ai against the best factory systems currently on the market. Slashdot reader schwit1 shares the report: If the self-driving car is the promised land, then today’s ever proliferating driver-assist features are the desert. Diminished claims and “it’s harder than we thought” mea culpas from self-driving’s loudest advocates suggest we’ll be…