test: improve test-cluster-net-send.js

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test, cluster

Description of change

This commit improves test-cluster-net-send as below;

• use port 0 instead of common.PORT
• use common.mustCall
• use assert.strictEqual instead of assert.equal
• change var to const or let (if possible)
• change function to arrow function

This is a part of Code And Learn at NodeFest 2016 Challenge nodejs/code-and-learn#58