lib/cpumask_kunit: log mask contents

For extra context, log the contents of the masks under test.  This
should help with finding out why a certain test fails.

Link: https://lore.kernel.org/lkml/CABVgOSkPXBc-PWk1zBZRQ_Tt+Sz1ruFHBj3ixojymZF=Vi4tpQ@mail.gmail.com/
Suggested-by: David Gow <davidgow@google.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Yury Norov <yury.norov@gmail.com>