test: clarify dropMembership() call

According to docs, dropMembership() is automatically called
by the kernel when the socket is closed, and most apps will
never need to call it. It's called here as a sanity check
only so let's note that with a comment.

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/io.js/pull/2062