When executing in multiversion mode, resmoke should retrieve the fixture files for the last-lts branch to assemble the multiversion cluster. These files should interact under the common interface defined in SERVER-54621, so fixtures themselves won't need multiversion-specific logic. In the case of sharded clusters or replica sets, the last-lts fixture file will be used for the cluster. (Future tickets will deal with upgrade/downgrade and version-specific edges.)

This ticket should also encompass establishing a method (either a guideline / playbook or automated tool) for changing the API from SERVER-54621 and backporting without temporary redness in the forward branch, to be used here and in the future.

One possibility involves creating a temporary fork of the back-branch containing the backport and pointing the forward-branch at it until the backport is complete.