Add rustdoc-json test to check new Attribute type

Author: GuillaumeGomez

tests/rustdoc-json/attrs/inner_vs_outer.rs

@@ -0,0 +1,8 @@
+#![no_std]
+
+//@ is "$.index[?(@.name=='foo')].attrs" '[{"content": "#[allow(unused)]", "is_inner": false}, {"content": "#[allow(dead_code)]", "is_inner": true}]'
+#[allow(unused)]
+pub mod foo {
+    #![allow(dead_code)]
+}
+