Be more explicit about #[test] dependency on std.

[nejucomo]

Another ticket, issue #1127 describes how dropping the std import will give confusing error messages when compiling with --test. In addition to improving those error messages, the tutorial should be very clear about this need.

The example already includes std:

use std;

fn twice(x: int) -> int { x + x }

#[test]
fn test_twice() {
    let mut i = -100;
    while i < 100 {
        assert twice(i) == 2 * i;
        i += 1;
    }
}

However the surrounding text does not emphasize that "use std" is necessary for tests. A complementary fix (and quicker) to #1127 is to simple change the sentence preceding the code snippet to something like this:

"Tests can be interspersed with other code, and annotated with the #[test] attribute. The #[test] attribute requires std to be in scope, so "use std" is required for compiling test code."

[steveklabnik]

Yep, as a newbie to Rust, I ran into this pretty hard too.

[brson]

use std is no longer necessary for tests. --test will link std automatically.