test: refactor test-dgram-send-callback-recursive

Just send 10 messages recursively and check that the send calls are
asynchronous by asserting that a `setImmediate` callback has been called
in-between. It avoids a race condition in the test when the recursive
limit is reached without having received at least 10 messages.

Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/5079