Mock Etherscan calls inside the etherscan.js tests

[manuelwedler]

Most of the tests in etherscan.js (not etherscanInstances.js) are intended to test our own handlers for the /verify/etherscan endpoint. This means we can mock the external HTTP call to the Etherscan API in many cases. This should make them faster and less dependent on their API being functional.

As far as I can see, we don't have a mocking library in the tests, yet. It might be worth adding one, like sinon.js for example or Nock for http mocking.