patch 9.0.1299: change for triggering incsearch not sufficiently tested

Problem:    Change for triggering incsearch not sufficiently tested.
Solution:   Add a test case.  Simplify the code. (closes #11971)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index c11f6ff..f2a763a 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1909,17 +1909,15 @@
   call assert_equal(':', g:entered)
   au! CmdlineChanged
 
+  autocmd CmdlineChanged : let g:log += [getcmdline()]
+
   let g:log = []
   cnoremap <F1> <Cmd>call setcmdline('ls')<CR>
-  autocmd CmdlineChanged : let g:log += [getcmdline()]
   call feedkeys(":\<F1>", 'xt')
   call assert_equal(['ls'], g:log)
-  unlet g:log
-  au! CmdlineChanged
   cunmap <F1>
 
   let g:log = []
-  autocmd CmdlineChanged : let g:log += [getcmdline()]
   call feedkeys(":sign \<Tab>\<Tab>\<C-N>\<C-P>\<S-Tab>\<S-Tab>\<Esc>", 'xt')
   call assert_equal([
         \ 's',
@@ -1950,6 +1948,22 @@
         \ 'sign unplace',
         \ ], g:log)
   set wildmenu& wildoptions&
+
+  let g:log = []
+  let @r = 'abc'
+  call feedkeys(":0\<C-R>r1\<C-R>\<C-O>r2\<C-R>\<C-R>r3\<Esc>", 'xt')
+  call assert_equal([
+        \ '0',
+        \ '0a',
+        \ '0ab',
+        \ '0abc',
+        \ '0abc1',
+        \ '0abc1abc',
+        \ '0abc1abc2',
+        \ '0abc1abc2abc',
+        \ '0abc1abc2abc3',
+        \ ], g:log)
+
   unlet g:log
   au! CmdlineChanged