Currently, we rely on mixed_version_replica_set.js to test the mixed version replica set test generation logic. This works correctly but will only run in the implicit multiversion build variant which is a daily builder. This could lead to a week or more for a failure of this test to get from being actually run to being turned into a BFG and then triaged into a BF.

Instead, it would be great to do 2 things:

• add a run-time check in replicaset.py to verify that the binaries actually started up match the configurations passed in through mixed_bin_versions.
• Move mixed_version_replica_set.js (it is currently in core) to a new test suite that gets run in the required builders as well as the implicit multiversion builder. This way, developers can detect breakages to multiversion test generation in their patch builds.