My first exposure to magazine road testing was reading Tom McCahill's tests in Mechanix Illustrated. If memory serves when he tested the 1955 Buick Century it did 0-60 in 10 seconds and we thought it was a rocket ship. How quaint.
Re Braking distance testing: Way back in the day, the MI State Police had a device at their driver training school that had three paint-shooting devices on the front bumper (aimed at the pavement, of course). The on-board instructor fired the first shot, the second fired from the brake pedal, and the third fired where and when the car stopped.
We had those devices in drivers ed (1970). I was lucky enough to put it to the test in front of the high school library. I can still see people looking out the library window to see what had just happened!
Fuel economy in the 50's and 60's? I recall the Shell commercials showing a professional driver in a car with a single gallon of gas mount externally above the hood. These drivers were masters. They treated the gas pedal as if it had a raw egg under it. Some of the mileage claims were pretty far fetched. 45 miles to a gallon in a full size American monster was unheard of yet these guys apparently could do so. The rest of the testing? All in the eyes of the beholder IMO.
What a big deal they were, too. There would be a brochure available for free at the gas station, and the winners of each class would have some in the car showroom. There was a Pure Oil performance trial each year, also widely followed. They were very showy about the scientific integrity of their test equipment.
In the early 50s motor trend did a suspension test by driving at high speed over railway tracks. Then they'd count the bounces afterward. Also a big feature was top speed as if you could compare cars that way. Cadillac in 54 did 113 miles per hour. Imagine if one of those tires had blown
Tom McCahill was my go-to every month, in Mechanix Illustrated, and when my father dug out his own collection of such magazines from when he was in his 20s (i.e. 1950's issues), when they were getting on towards 15 years old, I was in car-kid heaven. I enjoyed tests in British car magazines he'd bought, and reports by Floyd Clymer. (Look him up - fascinating life story).
I read an article once about testing in the early days of airplanes. To determine the maximum airspeed a plane could handle the test pilot would put the plane into a steep dive and keep a close eye on the airspeed indicator. When the wings came off he would make a mental note of the airspeed and parachute out.