rustdoc: set cfg(doctest) when collecting doctests

src/librustdoc/config.rs

@@ -351,6 +351,9 @@ impl Options {
                             .unwrap_or_else(|| PathBuf::from("doc"));
         let mut cfgs = matches.opt_strs("cfg");
         cfgs.push("rustdoc".to_string());
+        if should_test {
+            cfgs.push("doctest".to_string());
+        }
 
         let extension_css = matches.opt_str("e").map(|s| PathBuf::from(&s));