patch 8.2.5049: insufficient tests for autocommands

Problem:    Insufficient tests for autocommands.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #10507)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index cc7fe54..77965b3 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -1063,10 +1063,19 @@
   augroup END
   call feedkeys(":augroup X\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"augroup XTest.test", @:)
+
+  " group name completion in :autocmd
   call feedkeys(":au X\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"au XTest.test", @:)
+  call feedkeys(":au XTest.test\<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"au XTest.test", @:)
+
   augroup! XTest.test
 
+  " autocmd pattern completion
+  call feedkeys(":au BufEnter *.py\<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"au BufEnter *.py\t", @:)
+
   " completion for the :unlet command
   call feedkeys(":unlet one two\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"unlet one two", @:)