Written in response to a WP support forum question.

wordpress-redirect.php

<?php

/**
 * WordPress Conditional Redirect Function
 *
 * In this example, the conditional tag checks if the current page is a page with a slug of test and,
 * if true, replaces example.com with test.example.com and redirects.
 */
function my_custom_redirect() {
    if ( is_page( 'test' ) ) {
        global $wp;
        $current_url = add_query_arg( $wp->query_string, '', home_url( $wp->request ) );
        wp_redirect( preg_replace( 'example.com', 'test.example.com', $current_url ), 301 );
        exit;
    }
}
add_action( 'wp', 'my_custom_redirect' );

Revised the old code to use the WP core function wp_redirect.