test: refactor test-net-pipe-connect-errors

* var -> const
* try/catch -> assert.throws()
* assert.ok(fail) -> common.fail()
* assert.equal() -> assert.strictEqual()
* replace `exit` handler with `common.mustCall()`

PR-URL: https://github.com/nodejs/node/pull/8473


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>