How to exclude categories from get_categories() select list inside a widget admin panel

Per http://codex.wordpress.org/Class_Reference/WP_Query you would need to prefix the numerical identifier of the array with a minus-sign, such as $query = new WP_Query( 'cat=-12,-34,-56' );, which would exclude the categories with the identifier 12, 34 and 56.

You could achieve this by changing the get_cats function into the following:

function get_cats(){
    $exclude_cats = array(
        get_cat_ID('test1'),
        get_cat_ID('test2'),
        get_cat_ID('test3'),
    );
    foreach ($exclude_cats as $item) {
        $array[] = '-'.$item;
    }
    return $array;
}

Which returns the array exactly as before, except each item has a minus-sign prepended to it.