refactor(tests): all screen tests should use highlights

This is the first installment of a multi-PR series significantly
refactoring how highlights are being specified.

The end goal is to have a base set of 20 ish most common highlights,
and then specific files only need to add more groups to that as needed.

As a complicating factor, we also want to migrate to the new default
color scheme eventually. But by sharing a base set, that future PR
will hopefully be a lot smaller since a lot of tests will be migrated
just simply by updating the base set in place.

As a first step, fix the anti-pattern than Screen defaults to ignoring
highlights. Highlights are integral part of the screen state, not
something "extra" which we only test "sometimes". For now, we still
allow opt-out via the intentionally ugly

  screen._default_attr_ids = nil

The end goal is to get rid of all of these eventually (which will be
easier as part of the color scheme migration)

test/functional/ui/cursor_spec.lua

@@ -204,7 +204,7 @@ describe('ui/cursor', function()
     screen:expect {
       grid = [[
       ^                         |
-      ~                        |*3
+      {1:~                        }|*3
       test                     |
     ]],
       condition = function()
@@ -290,7 +290,7 @@ describe('ui/cursor', function()
     screen:expect {
       grid = [[
       ^                         |
-      ~                        |*3
+      {1:~                        }|*3
       test                     |
     ]],
       condition = function()