how can I make view_slideshow module thumbnail pager scrollable?
Views Slideshow JCarousel will suits your problem description best . Before Installing it you need to have views and views slideshow  enabled.One extra work you need to do is to create two folders in sites/all/libraries folder 

1) sites/all/libraries/jcarousel - download the .tar file from https://codeload.github.com/jsor/jcarousel/legacy.tar.gz/0.2.9 and extract here.

2) sites/all/libraries/jquery.cycle - save here the .js file you get form http://malsup.github.io/jquery.cycle.all.js link.

Now .


create a view of content of TEST (let here be the content type is TEST).
Select Format slide show and click settings . Now in Bottom Widget(you can select top widget also) check the pager checkbox.
It will ask all the requirements like Number of visible pager item and scroll items . Save it and you are done.