Created tests/rustdoc/extern subfolder to limit number of files at the top level

tests/rustdoc/extern/auxiliary/empty.rs

@@ -0,0 +1 @@
+// intentionally empty

tests/rustdoc/extern/auxiliary/variant-struct.rs

@@ -0,0 +1,5 @@
+pub enum Foo {
+    Bar {
+        qux: (),
+    }
+}