Add a const-generics folder to rustdoc tests

Author: varkor

src/test/rustdoc/generic-const.rs renamed to src/test/rustdoc/const-generics/const-impl.rs

@@ -1,7 +1,8 @@
+// ignore-tidy-linelength
+
 #![feature(const_generics)]
-#![crate_name = "foo"]
 
-// ignore-tidy-linelength
+#![crate_name = "foo"]
 
 pub enum Order {
     Sorted,