Adds a new zombienet-sdk test which verifies that elastic scaling works
correctly both with the MVP and the new RFC 103 implementation which
sends the core selector as a UMP signal.

Also enables the V2 receipts node feature for testnet genesis config.

Part of https://github.com/paritytech/polkadot-sdk/issues/5049

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Co-authored-by: Javier Viola <javier@parity.io>
Co-authored-by: Javier Viola <363911+pepoviola@users.noreply.github.com>