…test`.

Prior to this commit, new Rails applications would raise
`ActionView::Template::Error` if a mailer included a url built with a
`*_path` helper.

Since we already know [new apps will be served on `localhost:3000`][new
apps], we set this as the value in `development`.

In an effort to remain consistent with existing patters, we set the
`host` to `www.example.com` in `test.

[new apps]: https://github.com/rails/rails/blob/47300002db11d67d7b35103f5c429dad7dacdacd/README.md?plain=1#L81

…onments/development.rb.tt

Co-authored-by: Rafael Mendonça França <rafael@franca.dev>