Add fork_database unittests to cover best fork choice rules

[heifner]

Depends on AntelopeIO/leap#2125

AntelopeIO/leap#2125 Did not add test coverage for fork_database best fork choice rules.
Add unittest coverage of fork choices with multiple branches and scenarios.

[arhag]

We need to consider a worst-case scenario of finalizers split apart for different branches, then rejoin together, and need to reach consensus on one of the branches through their votes. It requires one of them to choose to vote weakly to join the other.