SERVER-36015 is removing system.indexes special handling in v4.2, except for the replication paths because a v4.0 mongod can still do inserts to <db>.system.indexes collections per SERVER-36714.

This ticket is adding integration testing specifically to ensure a v4.2 mongod continues to correctly handle v4.0 mongod's <db>.system.indexes inserts along its replication paths. The testing should be removed in the v4.4 dev cycle.

The multi-version testing should likely include testing of an <db>.system.indexes insert along the following replication paths:

• regular steady-state replication
• applyOps
• initial sync
• recoverable rollback
  I'd suggest a set up with a v4.0 primary and v4.2 secondary.