add doc to pg_test module in template (pgcentralfoundation#970)

Document that the `pg_test` module generated by the `cargo pgx new` template is a required item for a pgx extension if users ever want to use `cargo pgx test`.

Author: ChuckHend

cargo-pgx/src/templates/lib_rs

@@ -19,6 +19,8 @@ mod tests {{
 
 }}
 
+/// This module is required by `cargo pgx test` invocations. 
+/// It must be visible at the root of your extension crate.
 #[cfg(test)]
 pub mod pg_test {{
     pub fn setup(_options: Vec<&str>) {{