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.

src/methods-and-traits/exercise.md

@@ -18,7 +18,7 @@ implementing that same trait, adding behavior in the process. In the "Generics"
 segment this afternoon, we will see how to make the wrapper generic over the
 wrapped type.
 
-```rust,compile_fail
+```rust,compile_fail,editable
 {{#include exercise.rs:setup}}
 
 // TODO: Implement the `Logger` trait for `VerbosityFilter`.