[zstreamtest] Add compression determinism tests

[zstreamtest] Add compression determinism tests

* Run compression twice and check the compressed data is byte-identical.
  The compression loop had to be rewritten to ensure deteriminism. It is
  guaranteed by always making maximal forward progress.
* When nbWorkers > 0, change the number of workers 1/8 of the time.
* Run in single-pass mode 1/4 of the time.

I've run a few hundred thousand iterations of zstreamtest and have seen
no deteriminism issues so far. Before the zstdmt fix that skips the
single-pass shortcut non-determinism showed up in a few hundred
iterations.