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