Implement commitment chain forking test with different forking points.

[piotrm50]

Currently we have engine switching tests where we have either two competing chains, or three chains forking at the same slot (EngineSwitching_Tie).
We should implement a test in which we have three competing chains, each forking at different slot (like drawn below).
We should have 3 variations of that test, each with different winning chain.

                  /---------------- | C1
       / ------------------------ | C2
o------------------------------ | C3