vim-patch:8.1.2133: some tests fail when run as root

Problem:    Some tests fail when run as root.
Solution:   Add CheckNotRoot and use it. (James McCoy, closes vim/vim#5020)
07282f01da

Skip test_terminal.vim and test_viminfo.vim: N/A

src/nvim/testdir/shared.vim

@@ -343,6 +343,15 @@ func RunVimPiped(before, after, arguments, pipecmd)
   return 1
 endfunc
 
+func IsRoot()
+  if !has('unix')
+    return v:false
+  elseif $USER == 'root' || system('id -un') =~ '\<root\>'
+    return v:true
+  endif
+  return v:false
+endfunc
+
 " Get all messages but drop the maintainer entry.
 func GetMessages()
   redir => result