Fix mock example and move it to a file for testing (#1548)

I swear I tested this in an actual Rust file, but I somehow messed up
when copy-pasting it into the example. The code is now in the test file
so it will be correct.

Why is the indentation wrong? Because of

  https://github.com/rust-lang/mdBook/issues/1564

The indentation is not kept by the included content, which breaks the
Markdown.

src/testing/mockall.rs

@@ -1,3 +1,4 @@
+// ANCHOR: simple_example
 use std::time::Duration;
 
 #[mockall::automock]
@@ -6,8 +7,23 @@ pub trait Pet {
 }
 
 #[test]
-fn test_robot_pet() {
+fn test_robot_dog() {
     let mut mock_dog = MockPet::new();
     mock_dog.expect_is_hungry().return_const(true);
     assert_eq!(mock_dog.is_hungry(Duration::from_secs(10)), true);
 }
+// ANCHOR_END: simple_example
+
+// ANCHOR: extended_example
+#[test]
+fn test_robot_cat() {
+    let mut mock_cat = MockPet::new();
+    mock_cat
+        .expect_is_hungry()
+        .with(mockall::predicate::gt(Duration::from_secs(3 * 3600)))
+        .return_const(true);
+    mock_cat.expect_is_hungry().return_const(false);
+    assert_eq!(mock_cat.is_hungry(Duration::from_secs(1 * 3600)), false);
+    assert_eq!(mock_cat.is_hungry(Duration::from_secs(5 * 3600)), true);
+}
+// ANCHOR_END: extended_example

src/testing/mocking.md

@@ -8,7 +8,7 @@ For mocking, [Mockall] is a widely used library. You need to refactor your
 code to use traits, which you can then quickly mock:
 
 ```rust,ignore
-{{#include mockall.rs}}
+{{#include mockall.rs:simple_example}}
 ```
 
 [Mockall]: https://docs.rs/mockall/
@@ -40,20 +40,12 @@ code to use traits, which you can then quickly mock:
   existing Cargo project.
 
 - Mockall has a lot more functionality. In particular, you can set up
-  expectations which depend on the arguments passed:
+  expectations which depend on the arguments passed. Here we use this to mock a
+  cat which becomes hungry 3 hours after the last time it was fed:
 
-    ```rust,ignore
-    let mut mock_cat = MockPet::new();
-    mock_cat
-        .expect_is_hungry()
-        .with(mockall::predicate::gt(Duration::from_secs(3 * 3600)))
-        .return_const(true);
-    mock_cat
-        .expect_is_hungry()
-        .return_const(true);
-    assert_eq!(mock_cat.is_hungry(Duration::from_secs(1 * 3600)), false);
-    assert_eq!(mock_cat.is_hungry(Duration::from_secs(5 * 3600)), true);
-    ```
+```rust,ignore
+{{#include mockall.rs:extended_example}}
+```
 
 - You can use `.times(n)` to limit the number of times a mock method can be
   called to `n` --- the mock will automatically panic when dropped if this isn't