util: Add tests for fast integer division by constants

util: Add tests for fast integer division by constants

While I generally trust rediculousfish to have done his homework, we've
made some adjustments to suit the needs of mesa and it'd be good to
test those.  Also, there's no better place than unit tests to clearly
document the different edge cases of the different methods.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>