Mock endpoints during tests

[bitpshr]

The current test suite doesn't mock any endpoints during unit testing. Instead, they either hit real endpoints or skip request entirely. We should be setting nock.disableNetConnect(); in the unit test setup to ensure that none of our tests will send real network requests, but first we'll have to migrate each test that relies upon the network to use mocks.

EDITED TO ADD:

Here are a list of controllers that have tests in which we need to mock network requests:

• Mock network requests within all AssetsContractController tests #770
• Mock network requests within all CollectiblesController tests #769
• Mock network requests within all NetworkController tests #771
• Mock network requests within all PhishingController tests #772
• Mock network requests within all TransactionController tests #773