Add a real-browser test for the document preview #7

The "Print this?" screen now draws only the pages on screen, one at a time, but nothing tests that automatically. A later change could bring back slow previews without anyone noticing, because the tests today only cover the server side of previews (t14-preview-files.js) and uploads (t13-upload-retry.js).

What a test should check, in a phone-sized browser, using a PDF of about 40 pages:

• only the pages on screen are drawn at first
• scrolling to the end draws the last page
• tapping a page shows it bigger
• a file with mixed page sizes shows the right shapes
• a second file works straight after the first, with no errors in the browser console

A headless browser would need a new development-only package, so it mustn't end up in the installers. Please say in the issue which tool you plan to use before starting.