There are a couple things going on here.
First of all test_fn() is not a function in your jsfiddle. You will need to define it in order for it to be called.
Second of all you are setting the onclick attribute to be a string not a function. You would have to set it to be a function using the code below.
document.getElementById('xx').onclick = function() { test_fn(111,222) };
Here is an example of a working jsfiddle.
UPDATE
It looks like I misunderstood the question. If you would actually like to change the onclick attribute, as in, change what the HTML shows, you will want to use setAttribute(). So your code would be..
document.getElementById('xx').setAttribute( "onClick", "test_fn('111,222')" );
Here is the updated jsfiddle
As a side note, like nnnnnn says in a comment of the original post, using this you are passing 1 parameter through the function which is the string 111,222 when it seems that you might want to be passing two. If you would like to pass 111 as the first parameter and 222 as the second parameter, you would call the function with either test_fn(111,222) (passing the parameters as integers) or with test_fn('111','222') (passing the parameters as strings)