patch 8.2.0509: various code is not properly tested.

Problem:    various code is not properly tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5871)
diff --git a/src/testdir/test_viminfo.vim b/src/testdir/test_viminfo.vim
index 78e0f1a..044da0a 100644
--- a/src/testdir/test_viminfo.vim
+++ b/src/testdir/test_viminfo.vim
@@ -760,6 +760,11 @@
   call setfperm('Xviminfo', '--x------')
   call assert_fails('rviminfo Xviminfo', 'E195:')
   call delete('Xviminfo')
+
+  " Try to write the viminfo to a directory
+  call mkdir('Xdir')
+  call assert_fails('wviminfo Xdir', 'E886:')
+  call delete('Xdir', 'rf')
 endfunc
 
 " Test for writing to an existing viminfo file merges the file marks