Investigate weird RNG issue in dkg handover test

[cygnusv]

For some reason, the test RNG is producing repeated values in some cases, which made the handover test fail (by chance, the departing and incoming validators keypairs were the same, so after handover, transcript was the same 😅 ). Just by changing the order of invocation of the RNG "fixed" it, but this is of course not a solution. This failure doesn't affect the correctness of handover, but rather the affected test surfaced this issue.