patch 8.2.0203: :helptags and some other functionality not tested

Problem:    :helptags and some other functionality not tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5567)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 87c1f1f..cbd08c9 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -933,4 +933,13 @@
   tabclose
 endfunc
 
+" Test for the :only command
+func Test_window_only()
+  new
+  set modified
+  new
+  call assert_fails('only', 'E445:')
+  only!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab