Create global chai setup

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**Context**
Currently `chai` is directly imported on each test file, where it is then attached to the ethereum-waffle's `solidity` chai matcher (i.e.: [here](https://github.com/livepeer/protocol/blob/confluence/test/unit/Governor.js#L8-L9)). Furthermore, there are occurrences where it uses different assertion styles (i.e.: [here](
https://github.com/livepeer/protocol/blob/confluence/test/unit/Governor.js#L7) and [here](https://github.com/livepeer/protocol/blob/confluence/test/integration/PoolUpdatesWithHints.js#L125)).

**TODO**
- Create a global chai setup in its own separate file
- Replace `ethereum-waffle` with `chai-ethers`: the `ethereum-waffle` library is much broader in its scope than chai-ethers which, similarly to ethereum-waffle's solidity, only provides chai matchers to deal with ethereum testing. Removing `ethereum-waffle` will prevent potential misuse in the future should anyone use it for anything else other than attaching its matchers to chai.
- Enforce a consistent assertion style: since it seems that the most used construct is `expect`, we should remove all the occurrences of `assert`
- add [expectThrow](https://github.com/livepeer/protocol/blob/confluence/test/helpers/expectThrow.js) and [expectFail](https://github.com/livepeer/protocol/blob/confluence/test/helpers/expectFail.js) as custom assertions in the custom chai setup

Related to https://github.com/livepeer/protocol/issues/566

Start by reviewing the Chai imports and assertions in test/unit/Governor.js and test/integration/PoolUpdatesWithHints.js, then inspect test/helpers/expectThrow.js and test/helpers/expectFail.js. Create the separate global setup, migrate the matcher dependency and assertion usage, and verify that the existing test suite still passes with the custom assertions available globally.

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