Add a new #[allow(missing_docs)]

This commit adds a new `#[allow(missing_docs)]` attribute to supress a
new warning on rustc +beta for our test code. We do not really care
about doc comments there.

diesel/src/test_helpers.rs

@@ -1,3 +1,4 @@
+#![allow(missing_docs)] // test only module
 extern crate dotenvy;
 
 use crate::prelude::*;