Fix failing test after cloning repo for the first time.

Currently the tests will fail on a fresh checkout of the repo, but will fail the second time the tests are run. This is because the last test is not waiting until the I/O has finished in runFixture to make its final assertion.

This fixes that issue by adding an optional callback to runFixture.

How to reproduce:
- Check out a fresh copy of the repo followed by:
```
npm install
npm test
```
- Tests should fail. Repeat `npm test` and the tests will pass.
- `rm fixtures/assets/generated/390025aef74c5829.jpg` and the test will fail again.