I think my second point is more important-- The function doesn't implement "double-integer", it implements "multiply-all-numbers-together-and-then-by-2".. :)

The tests should be updated to check for functions that, when given more than one arg, do more than double the first, or just check that the function's arity is 1.

This is inefficient since partial creates a function that can take any number of args, plus it is incorrect, because it can take more than one arg (the function name being double-integer). I.e., your function could be passed 3 and 4, (double-integer 3 4) ; 24