Always show 4 items in the example list

If a library has lots of items then the first 3 should be shown, with
a count of how many more there are, for a total of 4 list items:
> a, b, c, and 23 more

If the library only has 4 items then all 4 should be shown, with
consistent use of conjunction and Oxford comma[1]:
> a, b, c, and d

This keeps the lengths of the examples nice and consistent.

1. We use an Oxford comma because it helps disambiguate when an area
itself has a comma or ‘and’ in it, for example ‘Armagh City, Banbridge
and Craigavon’

tests/app/main/views/test_broadcast.py

@@ -299,7 +299,7 @@ def test_choose_broadcast_library_page(
     ])
 
     assert normalize_spaces(page.select('.file-list-hint-large')[1].text) == (
-        'England, Northern Ireland, Scotland, Wales'
+        'England, Northern Ireland, Scotland, and Wales'
     )
 
     assert page.select_one('a.file-list-filename-large.govuk-link')['href'] == url_for(