Elastic scaling: add e2e test (#3929)

On top of https://github.com/paritytech/polkadot-sdk/pull/3879

I've also moved the previous test where we ensure multiple cores per
para doesn't break non elastic parachains.

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Signed-off-by: Andrei Sandu <[email protected]>
Co-authored-by: Javier Viola <[email protected]>