test: Replace global BufferList with local require

Removes assigning BufferList to a global in top level test-helper and adds explicit
require in the tests that need to access it.

packages/pg/test/buffer-list.js

@@ -1,5 +1,6 @@
 'use strict'
-global.BufferList = function () {
+
+const BufferList = function () {
   this.buffers = []
 }
 var p = BufferList.prototype

packages/pg/test/test-buffers.js

@@ -1,5 +1,6 @@
 'use strict'
 require('./test-helper')
+const BufferList = require('./buffer-list')
 // http://developer.postgresql.org/pgdocs/postgres/protocol-message-formats.html
 
 var buffers = {}

packages/pg/test/test-helper.js

@@ -3,7 +3,6 @@
 global.assert = require('assert')
 var sys = require('util')
 
-var BufferList = require('./buffer-list')
 const Suite = require('./suite')
 const args = require('./cli')
 

packages/pg/test/unit/client/md5-password-tests.js

@@ -1,5 +1,6 @@
 'use strict'
 var helper = require('./test-helper')
+const BufferList = require('../../buffer-list')
 var utils = require('../../../lib/utils')
 
 test('md5 authentication', function () {

packages/pg/test/unit/connection/inbound-parser-tests.js

@@ -1,5 +1,6 @@
 'use strict'
 require('./test-helper')
+const BufferList = require('../../buffer-list')
 var Connection = require('../../../lib/connection')
 var buffers = require('../../test-buffers')
 var PARSE = function (buffer) {