patch 8.1.1353: undo test fails on Mac

Problem:    Undo test fails on Mac.
Solution:   Expect "private" on the Mac.
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index ba1c861..1440660 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -442,7 +442,11 @@
 
   if isdirectory('/tmp')
     set undodir=/tmp
-    call assert_equal('/tmp/%tmp%file', undofile('///tmp/file'))
+    if has('osx')
+      call assert_equal('/tmp/%private%tmp%file', undofile('///tmp/file'))
+    else
+      call assert_equal('/tmp/%tmp%file', undofile('///tmp/file'))
+    endif
   endif
 
   set undodir&