--test_sharding_strategy=<explicit, disabled or forced=k where k is the number of shards to enforce>
Specify strategy for test sharding: 'explicit' to only use sharding if the 'shard_count' BUILD attribute is present. 'disabled' to never use test sharding. 'forced=k' to enforce 'k' shards for testing regardless of the 'shard_count' BUILD attribute.