Add epoch processing test for sync committee updates

tests/core/pyspec/eth2spec/test/lightclient_patch/epoch_processing/test_process_final_updates.py

@@ -0,0 +1,37 @@
+from eth2spec.test.context import (
+    PHASE0, PHASE1,
+    with_all_phases_except,
+    spec_state_test,
+)
+from eth2spec.test.helpers.state import transition_to
+from eth2spec.test.phase0.epoch_processing.run_epoch_process_base import (
+    run_epoch_processing_with,
+)
+
+
+@with_all_phases_except([PHASE0, PHASE1])
+@spec_state_test
+def test_sync_committees_progress(spec, state):
+    current_epoch = spec.get_current_epoch(state)
+    # NOTE: if not in the genesis epoch, period math below needs to be
+    # adjusted relative to the current epoch
+    assert current_epoch == 0
+
+    first_sync_committee = state.current_sync_committee
+    second_sync_committee = state.next_sync_committee
+
+    slot_at_end_of_current_period = spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD * spec.SLOTS_PER_EPOCH - 1
+    transition_to(spec, state, slot_at_end_of_current_period)
+
+    # Ensure assignments have not changed:
+    assert state.current_sync_committee == first_sync_committee
+    assert state.next_sync_committee == second_sync_committee
+
+    yield from run_epoch_processing_with(spec, state, 'process_final_updates')
+
+    # Can compute the third committee having computed final balances in the last epoch
+    # of this `EPOCHS_PER_SYNC_COMMITTEE_PERIOD`
+    third_sync_committee = spec.get_sync_committee(state, 2 * spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+
+    assert state.current_sync_committee == second_sync_committee
+    assert state.next_sync_committee == third_sync_committee