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/api/buffer_spec.lua

@@ -1744,12 +1744,11 @@ describe('api/buf', function()
       api.nvim_buf_set_text(0, 0, 0, 1, 3, { 'XXX', 'YYY' })
 
       screen:expect([[
-  XXX                 |
-  YYY                 |
-  ^                    |
-  ~                   |
-                      |
-
+        XXX                 |
+        YYY                 |
+        ^                    |
+        {1:~                   }|
+                            |
       ]])
     end)