Musa has the right approach here (from the comments). You can access the script tag like any other (you can use an ID to make it easy):
<script id='my_script_1' src="/path/file.js?test=1&data=2"></script>
Then just parse out the src and grab only the query string:
var js_lib = document.getElementById('my_script_1');
var js_url = js_lib.src;
var js_url_pieces = js_url.split('?');
alert(js_url_pieces[1]);
Note: I'm using split for simplicity's sake. You might want to use regex.
If you want reload the js file, just reset the source:
js_lib.src = js_url_pieces[0]+'?new query string';
I think that should work in most browsers.
Alternately, as others have mentioned, you might want to write more flexible functions or use global variables to achieve what you're trying to do through _GET vars.