promote-to-standard has unexpected behaviour on non-standard-candidate chains

[geoknee]

• It should be a no-op on standard chains.

• It should only modify the intended config file fields, it should not modify the hardfork times for instance.

[geoknee]

I think this command would benefit from some e2e tests. I noticed today that it also adds hardfork times to the config file, which I don't think we want.