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/token/ERC20/behaviors/ERC20Mintable.behavior.js

@@ -1,5 +1,6 @@
 const { assertRevert } = require('../../../helpers/assertRevert');
 const expectEvent = require('../../../helpers/expectEvent');
+const { ZERO_ADDRESS } = require('../../../helpers/constants');
 
 const BigNumber = web3.BigNumber;
 
@@ -8,8 +9,6 @@ require('chai')
   .should();
 
 function shouldBehaveLikeERC20Mintable (minter, [anyone]) {
-  const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
-
   describe('as a mintable token', function () {
     describe('mint', function () {
       const amount = 100;