test_caller_bcd: refactor addg6s test

test_caller_bcd: refactor addg6s test

This patch should vastly simplify and speed up the addg6s test. Most
importantly, we drop half adders and full adders, making use of the
fact that Python uses big integers directly. Also, we don't bother
generating all posible products of BCD numbers; instead, we simply
resort to random number generator. Note, however, that we only check
the numbers that are correct from BCD point of view.