isl_test.c: test_un_val: extract out val_check_equal
isl_test.c: test_un_val: extract out val_check_equal

This function will be reused in the next commit.

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

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