Add pre_epoch and post_epoch in epoch processing spec test

[ensi321]

Problem description

ethereum/consensus-specs#4155 adds pre_epoch and post_epoch states to spec test to verify states before and after epoch processing.

Update our spec test to test these too.

Solution description

Update epoch_processing.test.ts

Additional context

No response

[nflaig]

@ensi321 I don't think we wanna close this already but "partially resolve" wording used in https://github.com/ChainSafe/lodestar/pull/7919 will close this automatically after merging the PR