TestingBeaconNode funcs should return data matching spec-tests expectations

[iurii-ssv]

While implementing this adjustment - ssvlabs/ssv#2562 - we've discovered that in spec-tests BeaconNodeWrapped that relies on TestingBeaconNode (from ssv-spec repo) hardcodes the data it returns,

specifically for GetBeaconBlock endpoint:

• TestingBeaconNode returns hardcoded non-blinded blocks
• but, at the same time, some spec-tests have certain expectations on the block-roots for blinded blocks

This worked so far due to some "correct hardcoding" in place, but with ssvlabs/ssv#2562 the behavior in ssv repo changes - now, when we get a full block from the Beacon node (TestingBeaconNode), we are "creating" a blinded block ourselves (and SZZ-encode the result to use in QBFT consensus)

and so the spec-test mentioned above fails now,

Solution(s):

• perhaps we can do some more of elaborate hardcoding, but
• the correct general-purpose solution would require updating spec-tests such that the results TestingBeaconNode funcs/endpoints returns be parametrizable by every individual spec-test as needed (if spec-tests works with blinded block(s) - BeaconNodeWrapped would return blinded blocks, etc.)