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

They were starting to get unwieldy.

src/test/rustdoc/intra-doc/external-traits.rs

@@ -0,0 +1,12 @@
+// aux-build:intra-links-external-traits.rs
+// ignore-cross-compile
+
+#![crate_name = "outer"]
+#![deny(broken_intra_doc_links)]
+
+// using a trait that has intra-doc links on it from another crate (whether re-exporting or just
+// implementing it) used to give spurious resolution failure warnings
+
+extern crate intra_links_external_traits;
+
+pub use intra_links_external_traits::ThisTrait;