patch 8.2.0197: some Ex commands not sufficiently tested

Problem:    Some Ex commands not sufficiently tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5565)

src/testdir/test_help.vim

@@ -12,6 +12,18 @@ endfunc
 func Test_help_errors()
   call assert_fails('help doesnotexist', 'E149:')
   call assert_fails('help!', 'E478:')
+  if has('multi_lang')
+    call assert_fails('help help@xy', 'E661:')
+  endif
+
+  let save_hf = &helpfile
+  set helpfile=help_missing
+  help
+  call assert_equal(1, winnr('$'))
+  call assert_notequal('help', &buftype)
+  let &helpfile = save_hf
+
+  call assert_fails('help ' . repeat('a', 1048), 'E149:')
 
   new
   set keywordprg=:help