Add test helper to check balance difference

[nventuro]

It's been mentioned that we have the following pattern in multiple parts of our test suite:

const preBalance = await ethGetBalance(account);
await this.contract.withdrawPayments(account);
const postBalance = await ethGetBalance(count);

postBalance.sub(preBalance).should.be.bignumber.equal(value);

We should develop a test helper to take care of that (e.g. by receiving a function to await on, and then return the balance difference):

(await balanceDifference(account, () => this.contract.wihtdrawPayments(account)))
  .should.be.bignumber.equal(value);