message: Use the same `start` timestamp for all validators.
  Make sure it's a bit in the future.
diff: @@ -857,7 +857,7 @@ impl<DS: DeliveryStrategy> HighwayTestHarnessBuilder<DS> {
  TEST_MAX_ROUND_EXP,
  TEST_MIN_ROUND_EXP,
  TEST_END_HEIGHT,
  - Timestamp::now(),
  + Timestamp::zero(),
  Timestamp::zero(), // Length depends only on block number.
  TEST_ENDORSEMENT_EVIDENCE_LIMIT,
  );
  