The convert_to_and_from_sharded.js test exercises the scenario of adding a standalone shard to a cluster. The test adds the shard, moves chunks to the shard, then removes the shard, draining it completely. With the transition to shards as replica sets starting in 4.0, it may not be worth keeping this test.
Additionally, any code changes that may rely on a shard acting as a replica set may cause this test to break. Getting rid of or altering this test to conform with current supported practices will prevent future test problems.