Tests: fixed ClientHello with resending Initial QUIC packets. Previously it was rebuilt each time using distinct ClientHello.random resulting in different CRYPTO payload. As such, it led to TLS digest hash and derived secrets mismatch when resending Initial packet. Now ClientHello is built once and reused when resending Initial packets. Additionally, this required to preserve a generated secret value used in shared secret calculation as part of TLS key schedule. Previously it was regenerated when receiving a Retry packet, but this won't work with reused ClientHello as the resulting shared secrets won't match.