$wpdb->get_var not returning count

In your code it is actually counting the query you have inserted in $testargs. In your case you entered one. So it is giving you back one. If you need to get the count on WordPress users table then write the query like below-

global $wpdb;

$total_query = "SELECT COUNT(*) FROM {$wpdb->users} AS total_count";
$total = $wpdb->get_var( $total_query );