Migrate some tests away from concat-idents

`concat_idents!` is in the process of being removed, but a few things it
is used to test will still be relevant. Migrate these tests to something
other than `concat_idents`.

tests/rustdoc-js/big-result.rs

@@ -1,4 +1,3 @@
-#![feature(concat_idents)]
 #![allow(nonstandard_style)]
 /// Generate 250 items that all match the query, starting with the longest.
 /// Those long items should be dropped from the result set, and the short ones