make libmwaw unit tests valgrind-clean

Change-Id: I4bb8a1810f484f7b991e1eb3791f69cb9b744363
Reviewed-on: https://gerrit.libreoffice.org/12984
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>