Fix/#1355 test helper to check balance difference (#1368)

* signing prefix added

* Minor improvement

* Tests changed

* Successfully tested

* Minor improvements

* Minor improvements

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

This reverts commit a6889776.

* fixex #1355

* linting

* suggested changes

* Update BreakInvariantBounty.test.js

test/BreakInvariantBounty.test.js

 const { ethGetBalance, ethSendTransaction } = require('./helpers/web3');
 const { sendEther } = require('./helpers/sendTransaction');
+const { balanceDifference } = require('./helpers/balanceDiff');
 const expectEvent = require('./helpers/expectEvent');
 const { assertRevert } = require('./helpers/assertRevert');
 
@@ -72,12 +73,8 @@ contract('BreakInvariantBounty', function ([_, owner, researcher, anyone, nonTar
 
           it('sends the reward to the researcher', async function () {
             await this.bounty.claim(this.target.address, { from: anyone });
-
-            const researcherPreBalance = await ethGetBalance(researcher);
-            await this.bounty.withdrawPayments(researcher);
-            const researcherPostBalance = await ethGetBalance(researcher);
-
-            researcherPostBalance.sub(researcherPreBalance).should.be.bignumber.equal(reward);
+            (await balanceDifference(researcher, () => this.bounty.withdrawPayments(researcher)))
+              .should.be.bignumber.equal(reward);
             (await ethGetBalance(this.bounty.address)).should.be.bignumber.equal(0);
           });
 

test/helpers/balanceDiff.js

+async function balanceDifference (account, promise) {
+  const balanceBefore = web3.eth.getBalance(account);
+  await promise();
+  const balanceAfter = web3.eth.getBalance(account);
+  return balanceAfter.minus(balanceBefore);
+}
+
+module.exports = {
+  balanceDifference,
+};