test: avoid test timeouts on rpi

Generating 1024-bit primes on rpi test machines sometimes
causes timeouts. Avoid this situation by using 256-bit
primes when not running in FIPS mode.

Fixes: https://github.com/nodejs/node/issues/3881
PR-URL: https://github.com/nodejs/node/pull/3902


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <ben@strongloop.com>