Add missing spec tests for rewards/penalties logic

[ralexstokes]

It looks like we currently do not run the rewards/penalties spec tests during the epoch processing fixtures.

Tests here:
https://github.com/ethereum/eth2.0-spec-tests/tree/v0.12.1/tests/minimal/phase0/epoch_processing/rewards_and_penalties/pyspec_tests

And new tests were added in v0.12.x to more granularly test rewards calculations; tests here:

https://github.com/ethereum/eth2.0-spec-tests/tree/v0.12.1/tests/minimal/phase0/rewards/core/pyspec_tests

We will want to add both to our fixture suite.

[ralexstokes]

NOTE: We may have to refactor our rewards logic to expose some of the code required to run some of the new tests...