patch 8.0.0283: mode() does not indicate Insert mode completion

Problem:    The return value of mode() does not indicate that completion is
            active in Replace and Insert mode. (Zhen-Huan (Kenny) Hu)
Solution:   Add "c" or "x" for two kinds of completion. (Yegappan Lakshmanan,
            closes #1397)  Test some more modes.

src/testdir/test_mapping.vim

@@ -110,6 +110,8 @@ func Test_map_langmap()
   call feedkeys(":call append(line('$'), '+')\<CR>", "xt")
   call assert_equal('+', getline('$'))
 
+  iunmap a
+  iunmap c
   set nomodified
 endfunc
 
@@ -120,7 +122,7 @@ func Test_map_feedkeys()
   $-1
   call feedkeys("0qqdw.ifoo\<Esc>qj0@q\<Esc>", "xt")
   call assert_equal(['fooc d', 'fooc d'], getline(line('$') - 1, line('$')))
-  unmap .
+  nunmap .
   set nomodified
 endfunc