Add test for let blocks

testing/templates/let.html

@@ -0,0 +1 @@
+{% let v = s %}{{ v }}

testing/tests/vars.rs

@@ -0,0 +1,16 @@
+#[macro_use]
+extern crate askama;
+
+use askama::Template;
+
+#[derive(Template)]
+#[template(path = "let.html")]
+struct LetTemplate<'a> {
+    s: &'a str,
+}
+
+#[test]
+fn test_let() {
+    let t = LetTemplate { s: "foo" };
+    assert_eq!(t.render().unwrap(), "foo");
+}