[feature] Improve test utils for report on files count

[woshilapin]

Historically, in our test_utils framework, we test the number of files produced vs the number of files in the fixture and compare them. This sometimes leads to something like this kind of error.

This is not very helpful. So I changed it to a comparison of the 2 lists which would show something like this (more helpful I believe). Note that I used a BTreeSet instead of a Vec so the list is ordered and the diff much more helpful.

As a bonus, hidden files are also filtered. This was inconvenient when you're currently editing a file (like transfers.txt) because some editors like vim create a swap file (like .transfers.txt.swp).

[datanel]

much better indeed