test: fix flaky test-crypto-classes.js

On non-FIPS, we can instantiate DiffieHellman with 256 instead of 1024.
This should be quite a bit faster, and therefore prevent the timeouts.

PR-URL: https://github.com/nodejs/node/pull/15662
Fixes: https://github.com/nodejs/node/issues/15655


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>