cppunittest_filter_xslt: fix "filter_xslt unit test failed" for OSX <10.6

For now, just disable this test on OSX <10.6.

Change-Id: I5f41f0d0dc3d6c48d27f5276268af3b47a71c7fc
Reviewed-on: https://gerrit.libreoffice.org/10458
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>