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/editor/ctrl_c_spec.lua

@@ -66,8 +66,8 @@ describe('CTRL-C (mapped)', function()
     feed('i')
     screen:expect([[
       ^                                                    |
-      ~                                                   |*4
-      -- INSERT --                                        |
+      {1:~                                                   }|*4
+      {5:-- INSERT --}                                        |
     ]])
   end)
 
@@ -81,8 +81,8 @@ describe('CTRL-C (mapped)', function()
     feed('i')
     screen:expect([[
       ^                                                    |
-      ~                                                   |*4
-      -- INSERT --                                        |
+      {1:~                                                   }|*4
+      {5:-- INSERT --}                                        |
     ]])
   end)
 end)