Add test for trait toggle location

Author: jsha

src/test/rustdoc/toggle-trait-fn.rs

@@ -0,0 +1,7 @@
+#![crate_name = "foo"]
+
+// @has foo/trait.Foo.html
+// @has - '//details[@class="rustdoc-toggle"]//code' 'bar'
+pub trait Foo {
+    fn bar() -> ();
+}