test: refactor test-dgram-bind-shared-ports.js

Refactored the code to latest standards, where all var is
changed to const, functions are changed to arrow functions
and assert.equal chaned to assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/8582


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>