Finality testing

[djrtwo]

Add tests to explicitly test each of the 4 finality rules.
These are pretty heavy wrt timing but very important tests.

Note, built off of #988 so merge in #988 before review

[hwwhww]

The logic looks good to me!

Two possible refactoring approaches:

1. Moving the assertions of changed finalized_epoch to check_finality. For example:

   def check_finality(state,
                   prev_state,
                   current_justified_changed,
                   previous_justified_changed,
                   finalized_is_current_justified,
                   finalized_is_previous_justified):
   
       ...
   
       if finalized_is_current_justified:
           assert state.finalized_epoch == prev_state.current_justified_epoch
           assert state.finalized_root == prev_state.current_justified_root
       elif finalized_is_previous_justified:
           assert state.finalized_epoch == prev_state.previous_justified_epoch
           assert state.finalized_root == prev_state.previous_justified_root
       else:
           assert state.finalized_epoch == prev_state.finalized_epoch
           assert state.finalized_root == prev_state.finalized_root

2. As Decouple justification and finalization processing #925 decouples some justification and finalization, we can use old_previous_justified_epoch and old_current_justified_epoch in check_finality for improving readability when comparing with the spec.