Add unit tests to cow_wrapper for the move ctor and move assignment.

Change-Id: I82a5886ca7ae110985c7202125699cf95b6466d8
Reviewed-on: https://gerrit.libreoffice.org/18108Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>