Move `experiment` key under `fluxsite`

[SeanBryan51]

The experiment key in the config is specific to fluxsite tests so I think we should move it under the fluxsite key.

Currently experiment is a required key and fluxsite is an optional key. Since it will be possible to run spatial only tests, I'd prefer to keep fluxsite as an optional key. Moving experiment into fluxsite would then require setting a default value for the experiment key so that fluxsite stays optional. I propose we set the default to be the forty-two-site-test.