Move src/test/rustdoc intra-doc link tests into a subdirectory

They were starting to get unwieldy.

src/test/rustdoc/intra-doc/prim-methods-external-core.rs

@@ -0,0 +1,18 @@
+// aux-build:my-core.rs
+// build-aux-docs
+// ignore-cross-compile
+// ignore-windows
+// ignore-tidy-linelength
+
+#![deny(broken_intra_doc_links)]
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "rlib"]
+
+// @has prim_methods_external_core/index.html
+// @has - '//*[@id="main"]//a[@href="https://doc.rust-lang.org/nightly/std/primitive.char.html"]' 'char'
+// @has - '//*[@id="main"]//a[@href="https://doc.rust-lang.org/nightly/std/primitive.char.html#method.len_utf8"]' 'char::len_utf8'
+
+//! A [`char`] and its [`char::len_utf8`].
+
+extern crate my_core;