Be more consistent about tests vs. main (#2644)

The content slides all use `fn main`, with the exception of the testing
segment. But with this change, where it makes sense exercises use tests
instead, and not both tests and `fn main`.

A small change in `book.js` supports running tests when a code sample
does not have `fn main` but does have `#[test]`, so these work
naturally.

Fixes #1581.

third_party/rust-on-exercism/health-statistics.rs

@@ -50,13 +50,6 @@ impl User {
     }
 }
 
-// ANCHOR: main
-fn main() {
-    let bob = User::new(String::from("Bob"), 32, 155.2);
-    println!("I'm {} and my age is {}", bob.name, bob.age);
-}
-// ANCHOR_END: main
-
 // ANCHOR: tests
 #[test]
 fn test_visit() {