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/legacy/cmdline_spec.lua

@@ -93,8 +93,8 @@ describe('cmdline', function()
     feed_command('DoSomething')
     screen:expect([[
                                                                   |
-      ~                                                           |*2
-                                                                  |
+      {1:~                                                           }|*2
+      {3:                                                            }|
       Executing: DoSomething                                      |
       Executing: echo 'hello' |set ts=4 |let v = '123' |echo v    |
       hello                                                       |
@@ -102,7 +102,7 @@ describe('cmdline', function()
       Executing: let v = '123' |echo v                            |
       Executing: echo v                                           |
       123                                                         |
-      Press ENTER or type command to continue^                     |
+      {6:Press ENTER or type command to continue}^                     |
     ]])
   end)