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: #5487
closes: #14922

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/buffer.c b/src/buffer.c
index 58e9718..cbec9b9 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2429,6 +2429,7 @@
     clear_string_option(&buf->b_p_lop);
     clear_string_option(&buf->b_p_cinsd);
     clear_string_option(&buf->b_p_cinw);
+    clear_string_option(&buf->b_p_cot);
     clear_string_option(&buf->b_p_cpt);
 #ifdef FEAT_COMPL_FUNC
     clear_string_option(&buf->b_p_cfu);