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/unsafe-rust/exercise.md

@@ -51,7 +51,7 @@ The [Nomicon] also has a very useful chapter about FFI.
 Copy the code below to <https://play.rust-lang.org/> and fill in the missing
 functions and methods:
 
-```rust,should_panic
+```rust,should_panic,editable
 // TODO: remove this when you're done with your implementation.
 #![allow(unused_imports, unused_variables, dead_code)]