patch 9.1.0771: completion attribute hl_group is confusing

Problem:  Currently completion attribute hl_group is combined with
          all items, which is redundant and confusing with kind_hlgroup
Solution: Renamed to abbr_hlgroup and combine it only with the abbr item
          (glepnir).

closes: #15818

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index a4f6d02..3c748a4 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -1504,7 +1504,7 @@
   call StopVimInTerminal(buf)
 endfunc
 
-func Test_pum_user_hl_group()
+func Test_pum_user_abbr_hlgroup()
   CheckScreendump
   let lines =<< trim END
     func CompleteFunc( findstart, base )
@@ -1513,9 +1513,9 @@
       endif
       return {
             \ 'words': [
-            \ { 'word': 'aword1', 'menu': 'extra text 1', 'kind': 'W', 'hl_group': 'StrikeFake' },
+            \ { 'word': 'aword1', 'menu': 'extra text 1', 'kind': 'W', 'abbr_hlgroup': 'StrikeFake' },
             \ { 'word': 'aword2', 'menu': 'extra text 2', 'kind': 'W', },
-            \ { 'word': '你好', 'menu': 'extra text 3', 'kind': 'W', 'hl_group': 'StrikeFake' },
+            \ { 'word': '你好', 'menu': 'extra text 3', 'kind': 'W', 'abbr_hlgroup': 'StrikeFake' },
             \]}
     endfunc
     set completeopt=menu
@@ -1557,7 +1557,7 @@
       endif
       return {
             \ 'words': [
-            \ { 'word': 'aword1', 'menu': 'extra text 1', 'kind': 'variable', 'kind_hlgroup': 'KindVar', 'hl_group': 'StrikeFake' },
+            \ { 'word': 'aword1', 'menu': 'extra text 1', 'kind': 'variable', 'kind_hlgroup': 'KindVar', 'abbr_hlgroup': 'StrikeFake' },
             \ { 'word': 'aword2', 'menu': 'extra text 2', 'kind': 'function', 'kind_hlgroup': 'KindFunc' },
             \ { 'word': '你好', 'menu': 'extra text 3', 'kind': 'class', 'kind_hlgroup': 'KindClass'  },
             \]}