There currently already exists a sharding_last_stable_mongos_and_mixed_shards test suite that tests behavior using a mixed version sharded cluster where each shard has a different binary version but each shard’s internal replica set members will have the same binary versions. The FCV for the cluster is set to last-stable.

To complete mixed version test coverage, we will add a new test suite for each of the test cases that make use of ShardedClusterFixture. These test suites will be run as a 2-shard cluster with each having a replica set with 2 nodes. These test suites will run with the following configuration:
Shard1 - P(latest) ← S(last-stable)
Shard2 - P(last-stable) ← S(latest)
Config Server - P(latest) ← S(last-stable)