You are describing Chopsticks.
The plan is with one config file, it can fork the relaychain + multiple parachains. All without actually running any Substrate node. Smoldot will be used to execute the wasm runtime, which itself already supports running light clients for multiple chains so we know it will work.
Chopsticks is still very new but I already finding it super useful for many use cases. We intent to put resources into it and I do believe it can significantly changes how developers work with Polkadot ecosystem.
It is not just a testing tool, it is a customizable wasm runtime execution engine.
Those are for example other use cases that could be build on top of Chopsticks: Wasm view functions and Tool to dry run XCM transactions