Test setup helper

[nventuro]

The following lines are repeated in almost every single test file:

const BigNumber = web3.BigNumber;

require('chai')
  .use(require('chai-bignumber')(BigNumber))
  .should();

We should have a helper function that setups chai, probably exposing BigNumber along the way (which will also allow us to easily change that dependency in the future if we wish to). Some tests also need should, which should be exposed as well.