no_std examples won't build in mdbook test.

src/bare-metal/alloc.md

@@ -2,7 +2,7 @@
 
 To use `alloc` you must implement a global (heap) allocator.
 
-```rust,editable
+```rust,editable,compile_fail
 {{#include alloc-example/src/main.rs:Alloc}}
 ```
 

src/bare-metal/minimal.md

@@ -1,6 +1,6 @@
 # A minimal `no_std` program
 
-```rust,editable
+```rust,editable,compile_fail
 #![no_main]
 #![no_std]