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/control-flow-basics/exercise.md

@@ -30,7 +30,5 @@ initial `n`.
   todo!("Implement this")
 }
 
-{{#include exercise.rs:tests}}
-
 {{#include exercise.rs:main}}
 ```

src/control-flow-basics/exercise.rs

@@ -25,15 +25,14 @@ fn collatz_length(mut n: i32) -> u32 {
     len
 }
 
-// ANCHOR: tests
+// ANCHOR: main
+fn main() {
+    println!("Length: {}", collatz_length(11)); // should be 15
+}
+// ANCHOR_END: main
+// ANCHOR_END: solution
+
 #[test]
 fn test_collatz_length() {
     assert_eq!(collatz_length(11), 15);
 }
-// ANCHOR_END: tests
-
-// ANCHOR: main
-fn main() {
-    println!("Length: {}", collatz_length(11));
-}
-// ANCHOR_END: main