Gasoline octane is measured by performing a test in a standardized engine. "High test" gas means gas that, when tested, proved to have a high octane rating. In the days before self-service gas stations, the attendant would ask you "Regular or high-test?" (Nowadays I think the question would more likely be phrased "regular or premium;" and in the really old days, "Regular or ethyl?") So, "high-test" gasoline means high-octane gasoline that gives more power in high-performance engines. It has become a common joke to refer to (ordinary) coffee with caffeine, rather than decaf, as "high-test."

Premium gasoline, or high octane gasoline, is also called high test gasoline. High test coffee means very strong coffee.

It just means the darkest coffee (strong) because they need a lot of caffeine. its not commonly used. i had to look it up and i'm a native speaker. don't worry too much about it