patch 8.2.1803: a few failures are not tested

Problem:    A few failures are not tested.
Solution:   Test a few failures. (Dominique Pellé, closes #7075)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 026625e..9557828 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -756,6 +756,10 @@
   call feedkeys(":doautocmd User MyCmd a.c\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"doautocmd User MyCmd a.c\<C-A>", @:)
 
+  " completion of autocmd group after comma
+  call feedkeys(":doautocmd BufNew,BufEn\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"doautocmd BufNew,BufEnter", @:)
+
   " completion for the :augroup command
   augroup XTest
   augroup END