You have to define your function outside that context.

Maybe you could try:

function getResults (result) {
    console.log(result); // to see which data come from server
    return result.week_busyness;
}
function getError (result) {
    console.log(result); // to see which data come from server
}

then you call .Model() which always returns a promise.

test = new instance.web.Model('hr.employee')
       .query(["week_busyness"])
       .filter([["id", "=", employee_id]])
       .first().then(getResults); 

or

test = new instance.web.Model('hr.employee')
       .query(["week_busyness"])
       .filter([["id", "=", employee_id]])
       .first().done(getResults).fail(getError); 

If you use then(getResults) even if error occur you have to handle the returned data or error.

If you use .done(getResults), the function only called if call to server was ok

If you use .fail() you could catch error if happens.

If you need more information, try search for javascript promises

I hope it's help you.

Can you do this

test.done(function( value ) {
    console.log(value);
});