patch 8.2.0448: various functions not properly tested

Problem:    Various functions not properly tested.
Solution:   Add more tests, especially for failures. (Yegappan Lakshmanan,
            closes #5843)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 040324d..001bbac 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -525,6 +525,7 @@
   call assert_equal(2, tabpagenr('$'))
   call assert_equal(['Xb', 'Xa'], map(tabpagebuflist(1), 'bufname(v:val)'))
   call assert_equal(['Xc'      ], map(2->tabpagebuflist(), 'bufname(v:val)'))
+  call assert_equal(['Xc'      ], map(tabpagebuflist(), 'bufname(v:val)'))
 
   %bw!
 endfunc