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/references/exercise.md

@@ -7,7 +7,7 @@ minutes: 20
 We will create a few utility functions for 3-dimensional geometry, representing
 a point as `[f64;3]`. It is up to you to determine the function signatures.
 
-```rust,compile_fail
+```rust,compile_fail,editable
 // Calculate the magnitude of a vector by summing the squares of its coordinates
 // and taking the square root. Use the `sqrt()` method to calculate the square
 // root, like `v.sqrt()`.