Teams developing features behind FCV-gated feature flags may want to write tests which check the FCV upgrade step for their feature, i.e. a test that does something like:
1. Start a repl set with binary and FCV X
2. Restart the repl set with binary version Y (Y > X) and the feature flag enabled as of FCV Y
3. Upgrade the FCV and ensure the feature becoming enabled goes smoothly

We should add an example JS test that does this for reference, and update the README to link to it.

Original title: Create a way to enable a feature flag and gate it on an FCV for testing purposes
Original description: Right now in our JS tests there is no way to enable a feature flag and gate it on an FCV without changing it in IDL and enabling it for real. We can only use `setParameter` which only enables the feature flag but doesn't gate it on an FCV. It would be helpful to add this testing functionality so that features can properly test that their feature is enabled/disabled correctly on FCV upgrade/downgrade without having to enable their feature flag by default on IDL first.