make_webshots.py: side-effect - causes download of the poster pdfs

Attn @0xdevalias if you folks decide to make use of this tiny script either for performance timing or ideally beefed up for being the load script for scalability testing (https://github.com/sparkletown/sparkle/issues/1471)

After a few runs with huge variability in timing (after randomization of posters was introduced, which is great) I saw that I had bunch of PDFs on my local harddrive in the directory from which I was running the script!

Wild guess is that probably due to some options given to chromium or how selenium works, they were saved locally (which is good also since helped to identify the issue) while code was pulling PDF for the viewer in a poster room. So UI didn't finalize or as in other cases simply didn't react on "go back to posterhall" onClick until PDF preview widget didn't finish.