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/tuples-and-arrays/exercise.md

@@ -26,15 +26,10 @@ transpose a matrix (turn rows into columns):
 Copy the code below to <https://play.rust-lang.org/> and implement the function.
 This function only operates on 3x3 matrices.
 
-```rust,should_panic
-// TODO: remove this when you're done with your implementation.
-#![allow(unused_variables, dead_code)]
-
+```rust,should_panic,editable
 {{#include exercise.rs:transpose}}
     todo!()
 }
 
-{{#include exercise.rs:tests}}
-
 {{#include exercise.rs:main}}
 ```

src/tuples-and-arrays/exercise.rs

@@ -25,7 +25,23 @@ fn transpose(matrix: [[i32; 3]; 3]) -> [[i32; 3]; 3] {
     result
 }
 
-// ANCHOR: tests
+// ANCHOR: main
+fn main() {
+    let matrix = [
+        [101, 102, 103], // <-- the comment makes rustfmt add a newline
+        [201, 202, 203],
+        [301, 302, 303],
+    ];
+
+    println!("matrix: {:#?}", matrix);
+    let transposed = transpose(matrix);
+    println!("transposed: {:#?}", transposed);
+}
+// ANCHOR_END: main
+// ANCHOR_END: solution
+
+// This test does not appear in the exercise, as this is very early in the course, but it verifies
+// that the solution is correct.
 #[test]
 fn test_transpose() {
     let matrix = [
@@ -43,18 +59,3 @@ fn test_transpose() {
         ]
     );
 }
-// ANCHOR_END: tests
-
-// ANCHOR: main
-fn main() {
-    let matrix = [
-        [101, 102, 103], // <-- the comment makes rustfmt add a newline
-        [201, 202, 203],
-        [301, 302, 303],
-    ];
-
-    println!("matrix: {:#?}", matrix);
-    let transposed = transpose(matrix);
-    println!("transposed: {:#?}", transposed);
-}
-// ANCHOR_END: main