Add test with multiple deposit requests with same pubkey but different withdrawal credentials

[debjit-bw]

If a block contained multiple deposits with the same pubkey, Grandine would add them to BeaconState.pending_deposits with the same withdrawal credentials. link

This PR adds a test which checks that the withdrawal credentials associated with the same pubkey are retained, rather than being made the same. It randomly picks 2 deposit requests and their pubkeys are made the same as the top deposit.

fixes partially: #4054

[mkalinin]

Thank you for this PR!

[jtraglia]

This is really great, thank you! 😄

I pushed two small commits to help readability; notably a check which tells the reader that the top-up withdrawal credentials are indeed different.