add basic unary isl_multi_union_pw_aff tests
add basic unary isl_multi_union_pw_aff tests

These will be extended with tests for isl_multi_union_pw_aff objects
with non-trivial domains when support for those is added.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,nicolasvasilache/isl,nicolasvasilache/isl,inducer/isl-mirror,PollyLabs/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,inducer/isl-mirror,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,Meinersbur/isl,PollyLabs/isl,nicolasvasilache/isl