Rust has a nice solution for this though: tests can also be embedded in documentation comments:

    /// Adds one to the number given.
    ///
    /// # Examples
    ///
    /// ```
    /// let arg = 5;
    /// let answer = my_crate::add_one(arg);
    ///
    /// assert_eq!(6, answer);
    /// ```
    pub fn add_one(x: i32) -> i32 {
        x + 1
    }

More languages could adopt that idea, and a good StackOverflow answer would include those tests in the snippet. StackOverflow might even automatically run the tests and add a passing/failing badge!