cffi-tests: make compilation of 32-bit libtest optional.
cffi-tests: make compilation of 32-bit libtest optional.

Fixes lp#911557. Reported by Elias Pipping. Solution courtesy of
Daniel Herring.

3b/cffi,3b/cffi,3b/cffi,stassats/cffi,attila-lendvai/cffi,mcna/cffi,stassats/cffi,stassats/cffi,dkochmanski/cffi,mcna/cffi,dkochmanski/cffi,dkochmanski/cffi,cffi/cffi,cffi/cffi,attila-lendvai/cffi,cffi/cffi