Add rustdoc-json test for doc_cfg

Author: GuillaumeGomez

tests/rustdoc-json/doc_cfg.rs

@@ -0,0 +1,5 @@
+#![feature(doc_cfg)]
+
+//@ is "$.index[?(@.name=='f')].attrs" '[{"other": "#[doc(auto_cfg(hide(bar, values(none())), hide(blob, values(\"a\", \"14\"))))]"}]'
+#[doc(auto_cfg(hide(bar, values(none())), hide(blob, values("a", "14")),))]
+pub fn f() {}