added a test helper with common constants (#1400)

* signing prefix added

* Minor improvement

* Tests changed

* Successfully tested

* Minor improvements

* Minor improvements

* Revert "Dangling commas are now required. (#1359)"

This reverts commit a6889776f4.

* updates

* fixes #1206

test/drafts/ERC20Migrator.test.js

@@ -1,5 +1,5 @@
 const { assertRevert } = require('../helpers/assertRevert');
-
+const { ZERO_ADDRESS } = require('../helpers/constants');
 const ERC20Mock = artifacts.require('ERC20Mock');
 const ERC20Mintable = artifacts.require('ERC20Mintable');
 const ERC20Migrator = artifacts.require('ERC20Migrator');
@@ -11,8 +11,6 @@ require('chai')
   .should();
 
 contract('ERC20Migrator', function ([_, owner, recipient, anotherAccount]) {
-  const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
-
   const totalSupply = 200;
 
   it('reverts with a null legacy token address', async function () {