Bigint: incorrect division behavior

[hoshiumiarata]

It seems that the division of a bigint by a small integer, where one of the operands is negative, is not working correctly.

Non-bigint division behavior:

-10**10 / 3
 => -3333333334

Bigint division behavior:

-10**20 / 3
 => -33333333333333333332

The result should be -33333333333333333334 (Ruby 3.3 and mruby 3.3.0 also give this result).

The problem is probably caused by not correctly handling rounding in the division algorithm.

Test cases are placed in this pull request: #6444