elastic scaling RFC 103 end-to-end tests (#6452)

Adds two new zombienet-sdk tests:

- one 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.
- one which spawns a parachain with two collators, but only one of them
is using v2 receipts. The parachain will still make progress but without
full throughput

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

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