test: fix non-determinism in test-crypto-domains

The test implicitly assumed that crypto operations complete in the same
order as they are started but, because they go round-trip through the
thread pool, there is no such guarantee.  Enforce proper sequencing.

Fixes node-forward/node#22.

PR-URL: https://github.com/node-forward/node/pull/23


Reviewed-By: Fedor Indutny <fedor@indutny.com>