api/ui: add tests for popupmenu_external events

update screen.lua to use new style nvim_ui_attach

test/functional/terminal/scrollback_spec.lua

@@ -331,7 +331,7 @@ describe('terminal prints more lines than the screen height and exits', function
   it('will push extra lines to scrollback', function()
     clear()
     local screen = Screen.new(50, 7)
-    screen:attach(false)
+    screen:attach({rgb=false})
     execute('call termopen(["'..nvim_dir..'/tty-test", "10"]) | startinsert')
     wait()
     screen:expect([[