Find case for `__prod2_2level_upper_bound<prod_policy>(in_0, in_1, op)` is the best approximation

[SSoelvsten]

The function __prod2_2level_upper_bound<prod_policy>(in_0, in_1, op) in prod2 used to derive an upper bound by combining both 1-level and 2-level cuts information does not in our unit tests beat the purely 2-level cut version. The N-Queens problem shows this upper bound indeed can be the most tight one.

So, we should find a minimal example to show it is possible (and add it as a unit test).