Stop warning when doc testing proc macro crates

Add the actually_rustdoc option to the session that
is passed  to phase_2_configure_and_expand function
inside the rustdoc test module.

This prevents the warning code from triggering when
parsing proc_macro_derive attributes, just like when
`--test` is normally invoked.

src/librustdoc/test.rs

@@ -67,6 +67,7 @@ pub fn run(input: &str,
         crate_types: vec![config::CrateTypeDylib],
         externs: externs.clone(),
         unstable_features: UnstableFeatures::from_environment(),
+        actually_rustdoc: true,
         ..config::basic_options().clone()
     };