librustc: Add some missing use core::prelude::*; in the test cases

src/librustdoc/text_pass.rs

@@ -139,6 +139,8 @@ fn fold_impl(
 
 #[cfg(test)]
 mod test {
+    use core::prelude::*;
+
     use astsrv;
     use attr_pass;
     use desc_to_brief_pass;