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

(cherry picked from commit 6ae041bc)

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,
+};