Challenge 1: Testing subtract — Possible Solution
====================================================================

pub fn subtract(a: i32, b: i32) -> i32 {
    a - b
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_subtracts_two_numbers() {
        assert_eq!(subtract(10, 3), 7);
    }

    #[test]
    fn subtracting_a_number_from_itself_is_zero() {
        assert_eq!(subtract(5, 5), 0);
    }
}

WHY THIS WORKS AS AN ANSWER
------------------------------
The tests module is marked #[cfg(test)], meaning it only compiles
during test runs and never bloats a normal build — this chapter's own
core convention for organizing unit tests, and directly avoids the
warn-box's forgetting-#[cfg(test)] gotcha.

use super::* brings subtract (defined in the parent module) into
scope inside tests — necessary since subtract is otherwise only
visible in the module it was actually defined in.

Two separate #[test] functions cover two genuinely different cases:
a normal subtraction (10 - 3 = 7) and the specific edge case of
subtracting a number from itself, which should always yield zero
regardless of the number's actual value — a small but real edge case
worth its own explicit test, similar in spirit to how earlier
chapters treated zero/edge-case inputs as worth testing separately
from the "typical" case.