test: rename test-regress-nodejsGH-1531

Rename the test appropriately alongside mentioning the subsystem
Also, make a few basic changes to make sure the test conforms
to the standard test structure

Refs: nodejs#19105
Refs: https://github.com/nodejs/node/blob/master/doc/guides/writing-tests.md#test-structure

Author: ryzokuken

test/parallel/test-regress-GH-1531.js renamed to test/parallel/test-http-request-agent.js

@@ -1,5 +1,6 @@
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 
 // This test ensures that a http request callback is called
 // when the agent option is set
@@ -8,8 +9,6 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
-const fixtures = require('../common/fixtures');
-
 const https = require('https');
 
 const options = {