Add test for new implementations section title

Author: GuillaumeGomez

src/test/rustdoc/struct-implementations-title.rs

@@ -0,0 +1,9 @@
+#![crate_name = "foo"]
+
+pub struct Struc;
+
+// @has foo/struct.Struc.html
+// @has - '//*[@id="main"]/h2[@id="implementations"]' "Implementations"
+impl Struc {
+    pub const S: u64 = 0;
+}