BEEFY: `expect_validator_set()` fix (#2716)

Fixes #https://github.com/paritytech/polkadot-sdk/issues/2699

Modifying `expect_validator_set()` in order to be able to walk back
until block 0. The chain state at block 0 is available even if we use
`--sync fast` or `--sync warp`. This way we can retrieve the initial
authority set even when BEEFY genesis is 1 and there is no authority
change entry in the headers log.

Credits to @acatangiu

 for the solution

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Co-authored-by: Adrian Catangiu <[email protected]>