Fix statement distribution benchmark (#6369)

I've broken this test with
https://github.com/paritytech/polkadot-sdk/pull/5883 and this is the
fix.

The benchmark is now updated to use proper core index and session index
for the generated candidates.

TODO:
- [ ] <del> PRDoc </del>

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Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>