I would send your routine bit patterns that might be likely to trigger
errors, ie:

        1000000000000000000000
        0100000000000000000000
        0010000000000000000000
        0001000000000000000000
                :
        1100000000000000000000
        0110000000000000000000
                :
        1110000000000000000000
        0111000000000000000000
                :

You should test operands/results that start with a 0 and a 1 in every
(possible) bit position, preferrably also checking all possible permutations
for a single bit (0/0 --> 0, 0/0 -> 1, 0/1 -> 0, 0/1 -> 1, etc (depending on
the nearby bits and carries, etc, of course.)

In addition to 12345679* (9*d) -> ddddddddd, there is also
37037037037... * (3*d) -> dddddddd...  (ie more digits.)

You can also run some series approximation involving multiplication and
division, and see if it comes out pretty close.  (ie calculate PI*2^63)

BillW