Prevent WordPress from Rewriting URL for one template

You can use an internal rewrite instead of .htaccess:

function wpd_service_rewrite() {
    add_rewrite_rule(
        '^service/testing-tool/([a-z0-9.]+)$',
        'index.php?pagename=service/results',
        'top'
    );
}
add_action( 'init', 'wpd_service_rewrite' );

Don’t forget to flush rewrite rules after adding / changing them.