When `cfg.package` is already wrapped, and wrapped without the
`ExtensionSettings` key set, this would always add that key, even if its
value was blank. This would result in `cfg.finalPackage` being a
functionally-identical, but differently-input-addressed package. This is
generally undesirable as it may result in multiple derivations being
built, and also if the value of `cfg.package` is expected to be
unchanged by the user (e.g. because they want it to be consistent
between NixOS and HM configuration).
Add a test to ensure this does not regress in the default case. Only
test on newish stateVersion since the logic for `isWrapped` differs on
older versions.
