TRPL: Add rust Marker to Some Code Block

This adds strictly more information to the source files and reduces the
need for customized tooling to render the book.

(While this should not change the output of _rustbook_, it is very
useful when rendering the sources with external tools like Pandoc.)

src/doc/trpl/testing.md

@@ -195,7 +195,7 @@ parameter can be added to the `should_panic` attribute. The test harness will
 make sure that the failure message contains the provided text. A safer version
 of the example above would be:
 
-```
+```rust
 #[test]
 #[should_panic(expected = "assertion failed")]
 fn it_works() {