Development: add ability to mock states

[sohkai]

It would be very, very useful to have a way to easily mock dispute state.

For example, testing the final appeal round of a dispute is incredibly difficult and time consuming today.

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There may be a variety of ways to tackling this that we can research:

• Mock graphQL queries
• Override specific parts of graphQL queries
• Scripts to set up chain environment

My preference would be to see if we can do this at the graphQL layer, as scaffolding a chain and bringing it down would be time consuming. Doing it at the query layer would also likely allow us to easily manipulate the frontend from the frontend itself in the future.

The con to doing it at the query layer is that we wouldn't be able to run any action, but this is acceptable to me; the state transitions of the subgraph are something we should test separately.