In the following add/remove shard suites, we have random balancing enabled:

• aggregation_unsplittable_collections_on_random_shard_passthrough_with_config_transitions_and_add_remove_shard
• *concurrency_sharded*with_config_transitions_and_add_remove_shard
• jstestfuzz_sharded_with_config_transitions_and_add_remove_shard

In these, though, we have random balancing disabled:

• sharding_jscore_passthrough_with_config_transitions_and_add_remove_shard
• sharded_collections_jscore_passthrough_with_config_transitions_and_add_remove_shard

We should investigate whether we can enable random balancing in those 2 passthrough suites as having this enabled makes it more likely that an added shard is actually used.