Similar to resmoke fixtures, don't pass in any feature flag options to servers.js if not starting latest mongo* processes.

We'll have to manually check the version and add if-else conditionals for feature flags to use since shell fixtures don't have the luxury (and shouldn't) use fixture definitions from the old version's commit like resmoke fixture.

Add a variant that runs the implicit multiversion tests with all feature flags; it'll be identical to the current "implicit multiversion" variant.