test: add buffer alignment regression tests

Buffers instances can have arbitrary alignment. `node-ffi` depends on
this. Add some regression tests to ensure we don't break this in the
future.

PR-URL: https://github.com/nodejs/node/pull/5752


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>