She was testing for lipids (fats and oils).

Why ethanol means lipids

In school biology, using ethanol on a food sample is the classic emulsion test for lipids. The food is mixed with ethanol, then the clear solution is poured into water; a cloudy white emulsion shows that fats/oils are present. That’s why if Jenny used ethanol to test a food sample, the food group she was testing for was lipids.

So, the direct answer to
“jenny was testing a food sample using ethanol. which food group was she testing for?”
is: lipids (fats).

TL;DR: Ethanol is used in the emulsion test, which detects lipids (fats and oils) in food.