The random test generates [] as the empty array instead of the specified [[]] from the description.
The empty array is already manually tested above so it doesn't need to be retested here.

Is this the idiomatic way to do fuzzy equality in Rust?

LGTM

Not perfect but good fun. I ran into an issue with the input "(tan (* 2 x))" because my code output this:
     (/ 2 (^ (cos (* 2 x)) 2))
instead of this:
(* 2 (/ 1 (^ (cos (* 2 x)) 2)))

The issue was that the differentiation rule I used for tan(f(x)) rule was
f'(x) / ( cos(f(x))^2 )