vim-patch:9.1.0469: Cannot have buffer-local value for 'completeopt'

Problem:  Cannot have buffer-local value for 'completeopt'
          (Nick Jensen).
Solution: Make 'completeopt' global-local (zeertzjq).

Also for some reason test Test_ColonEight_MultiByte seems to be failing
sporadically now. Let's mark it as flaky.

fixes: vim/vim#5487
closes: vim/vim#14922

529b9ad62a

src/nvim/options.lua

@@ -1466,7 +1466,7 @@ return {
       expand_cb = 'expand_set_completeopt',
       full_name = 'completeopt',
       list = 'onecomma',
-      scope = { 'global' },
+      scope = { 'global', 'buffer' },
       short_desc = N_('options for Insert mode completion'),
       type = 'string',
       varname = 'p_cot',