fix experimental-ump-signals tests (#6214)
Resolves https://github.com/paritytech/polkadot-sdk/issues/6200 Also sets the feature on the rococo-parachain. Will be useful for zombienet testing
Resolves https://github.com/paritytech/polkadot-sdk/issues/6200 Also sets the feature on the rococo-parachain. Will be useful for zombienet testing