test: refactor test-net-pingpong

* var -> const.
* Verify that callbacks are called with common.mustCall.
* Replace usage of deprecated `server.connections`.
* Use common.fail instead of rethrowing errors.
* Remove console.log statements.
* assert.equal -> assert.strictEqual.
* Correct order of arguments in assert.strictEqual.

PR-URL: https://github.com/nodejs/node/pull/9812


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>