* It's a locally created test: Individual schools or teachers might devise their own fitness assessments and give them informal names like "zipper test." Without more context (like the school or region where you heard about it), it's impossible to know what it entails.
* It's a misremembered name: The name might be a misinterpretation or a nickname for a different fitness test that focuses on flexibility or range of motion, perhaps involving touching fingers to toes or performing a similar movement.
To find out what the "zipper test" is, you'd need to ask the person who mentioned it or check with the physical education department of the school where it's supposedly used.