When the fail point balancerShouldReturnRandomMigrations is enabled (only possible with test commands enabled), the balancer will use _shardsvrReshardCollection to move unsharded collections randomly throughout the cluster. SERVER-91090 avoids moveCollection and unshardCollection transiently failing for small capped collections by setting an explicit shard distribution in their resharding commands. We can make the same change to the balancer's resharding command (or possibly make it use moveCollection to avoid duplication) to prevent the same errors, which can trigger a tassert.