In tests, the private key was put into the validator deposit's withdraw
credentials so that it can be recovered later. This leads to problems
when creating the validators through other means that do not put the key
there. In general, mock private keys only depend on the validator index,
though, and because it is clear what the index of a validator is, it is
not actually needed to put the key into the credentials.
Etan Kissling
tersec
Zahary Karadjov