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/pattern-matching/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
+[lib]
 name = "eval"
 path = "exercise.rs"

src/pattern-matching/exercise.md

@@ -46,7 +46,7 @@ evaluate to `85`. We represent this as a much bigger tree:
 
 In code, we will represent the tree with two types:
 
-```rust,editable
+```rust
 {{#include exercise.rs:Operation}}
 
 {{#include exercise.rs:Expression}}

src/pattern-matching/exercise.rs

@@ -127,13 +127,3 @@ fn test_zeros() {
     );
 }
 // ANCHOR_END: tests
-
-fn main() {
-    let expr = Expression::Op {
-        op: Operation::Div,
-        left: Box::new(Expression::Value(10)),
-        right: Box::new(Expression::Value(2)),
-    };
-    println!("expr: {expr:?}");
-    println!("result: {:?}", eval(expr));
-}