It is best to have deterministic fuzz targets, so that if a failure
occurs, it can be easily reproduced.
This commit swaps the cryptographically secure RNG for a deterministic
one seeded from fuzzer input.
Move the functionality directly into completeHandshake instead. If a
failure does happen at any point during the handshake, it is beneficial
to know which line it happens on for debugging. The helper function was
hiding this information.
