Add integration test for include!

integration-tests/templates/include.out

@@ -0,0 +1,7 @@
+
+  INCLUDED: foo
+
+
+  INCLUDED: bar
+
+

integration-tests/templates/include.stpl

@@ -0,0 +1,3 @@
+<% for s in strs { %>
+  <% include!("included.stpl"); %>
+<% } %>

integration-tests/templates/included.stpl

@@ -0,0 +1 @@
+INCLUDED: <%= s %>

integration-tests/tests/basic.rs

@@ -73,3 +73,14 @@ struct CustomDelimiter;
 fn custom_delimiter() {
     assert_render("custom_delimiter", CustomDelimiter);
 }
+
+#[derive(TemplateOnce)]
+#[template(path = "include.stpl")]
+struct Include<'a> {
+    strs: &'a [&'a str],
+}
+
+#[test]
+fn test_include() {
+    assert_render("include", Include { strs: &["foo", "bar"] });
+}