test: refactor test-net-server-max-connections

The test timed out on Windows in CI. Made the following changes:

* reduced total connections from 200 to 20
* var -> const
* string concatenation -> templates
* assert.equal -> assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/8931


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>