test: cache lazy properties, fix style nits

inFreeBSDJail involves an execSync() and is used by localhost_ipv4 so
will be unnecessarily expensive, so cache both values and reuse
rather than re-evaluate each time.

Renamed localhost_ipv4 to localhostIPv4 for style consistency.

PR-URL: https://github.com/iojs/io.js/pull/1196


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>