The following multiVersion tests are currently set to start a shard as a standalone instead of a replica set. This was done to support backwards compatibility with older versions. Because standalone shards became unsupported in 4.0 and the current version is 4.2, it is now possible to start shards as replica sets in the following tests.

2_test_launching_cluster.js
balancer_multiVersion_detect.js
migration_between_mixed_FCV_mixed_version_mongods.js
migration_between_mixed_version_mongods.js
new_mongos_old_mongod_wire_version_clash.js
upgrade_cluster.js