How to add parameter to nginx query_string during redirect

I want to add one parameter to $query string. Now I have this config

server {
    server_name example.com;
    listen 80;

    return 301 http://test.example.com$request_uri;
}

But I want to add test=1 into query. There are use cases:

http://example.com =>
    http://test.example.com?test=1

http://example.com/example =>
    http://test.example.com/example?test=1

http://example.com/example/?var=1 =>
    http://test.example.com/example/?var=1&test=1

I tried to split $request_uri to $uri and $query_param and got

return 301 http://test.example.com$uri$query_string;

But I cannot just add test=1 into this string. If I add ?test=1 so in the third case I will get http://test.example.com/example/?var=1?test=1. If I add &test=1 there is in the first and second cases I will get http://test.example.com&test=1 and http://test.example.com/example&test=1.

Now I check my $query_string by this rules:

if ($query_string ~ ^$) {
    return 301 http://test.example.com$uri?test=test;
}

if ($query_string ~ ^.+$) {
    return 301 http://test.example.com$uri?$query_string&test=test;
}

But I think there is more tidy decision for this task.