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/terminal/scrollback_spec.lua

@@ -361,16 +361,19 @@ describe(':terminal prints more lines than the screen height and exits', functio
       line8                         |
       line9                         |
                                     |
-      [Process exited 0]            |
-      -- TERMINAL --                |
+      [Process exited 0]{2: }           |
+      {5:-- TERMINAL --}                |
     ]])
     feed('<cr>')
     -- closes the buffer correctly after pressing a key
-    screen:expect([[
+    screen:expect {
+      grid = [[
       ^                              |
-      ~                             |*5
+      {1:~                             }|*5
                                     |
-    ]])
+    ]],
+      attr_ids = { [1] = { foreground = 12 } },
+    }
   end)
 end)