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

(cherry picked from commit 58a42443df)

test/token/ERC20/ERC20.test.js

@@ -1,5 +1,6 @@
 const { assertRevert } = require('../../helpers/assertRevert');
 const expectEvent = require('../../helpers/expectEvent');
+const { ZERO_ADDRESS } = require('../../helpers/constants');
 
 const ERC20 = artifacts.require('ERC20Mock');
 
@@ -10,8 +11,6 @@ require('chai')
   .should();
 
 contract('ERC20', function ([_, owner, recipient, anotherAccount]) {
-  const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
-
   beforeEach(async function () {
     this.token = await ERC20.new(owner, 100);
   });