Add test to prevent src link regression

Author: GuillaumeGomez

src/test/rustdoc/ensure-src-link.rs

@@ -0,0 +1,6 @@
+#![crate_name = "foo"]
+
+// This test ensures that the [src] link is present on traits items.
+
+// @has foo/trait.Iterator.html '//h3[@id="method.zip"]/a[@class="srclink"]' "[src]"
+pub use std::iter::Iterator;