crypto/rand: make Prime not deterministic for a fixed input stream
rand.Prime does not guarantee the precise prime selection algorithm as
part of its contract. For example, it changed slightly in CL 387554. We
want to ensure that no tests come to rely on it staying the same, so
just like other cryptographic functions that use randomness in an
unspecified way (ECDSA signing, RSA PKCS #1 v1.5 encryption, RSA key
generation), make it randomly read an extra byte or not.