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/buffer_defs.h

@@ -533,6 +533,8 @@ struct file_buffer {
   char *b_p_cinsd;              ///< 'cinscopedecls'
   char *b_p_com;                ///< 'comments'
   char *b_p_cms;                ///< 'commentstring'
+  char *b_p_cot;                ///< 'completeopt' local value
+  unsigned b_cot_flags;         ///< flags for 'completeopt'
   char *b_p_cpt;                ///< 'complete'
 #ifdef BACKSLASH_IN_FILENAME
   char *b_p_csl;                ///< 'completeslash'