CppunitTest_sw_rtfexport: enable on MACOSX

Change-Id: Id125f7b2293f7813391ca583f60bcbf53f455afc
Reviewed-on: https://gerrit.libreoffice.org/8562
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>