[Core apps tests] Files (Nautilus)

Some input from @antoniof on what we could test in Nautilus:

• Mobile form factor
• Visibility and width of the sidebar at specific window sizes.

  We had cases where, while making the window narrower, the grid view would have switch from 3 to 2 columns just for a small interval, before the sidebar collapsed and, suddenly, it was 4 or 5 columns. But there are so many things that can change the result and require these magic values to be tweaked again! And I guess openQA is great at noticing if something is a couple of pixels off

...