Fix failing tests

Use relative symlinks instead of absolute ones.

Reported-by: Nick Dokos <ndokos@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2018-02/msg00241.html>

testing/examples/pub-symlink/link.org

@@ -1,2 +1 @@
-# Time-stamp: <2018-02-13 10:12:03 ngz>
-symlink
+Symlink

testing/examples/pub/link

@@ -1 +1 @@
-/home/ngz/dev/org-mode/testing/examples/pub-symlink
+../pub-symlink