I found this answer here (http://wordpress.stackexchange.com/a/166375/10099) and thought I’d reiterate since it may come in handy if you are developing and don’t want to deal with managing login information.

function my_autologin() {
    if (!is_user_logged_in()) {
        $uid = 'autologinuser';
        $user = get_userdatabylogin( $uid );
        wp_set_auth_cookie($user->ID);
        wp_set_current_user($user->ID);
        do_action('wp_login', $user_login);  // optional
    }
}
add_action('init','my_autologin');

Enjoy.