Use broccoli-test-helper to make tests easier to work with

[lukemelia]

• Also update mocha to support async/await tests

In the time since tests were first written for this plugin, broccoli-test-helper was created, which allows describing input and expected output directories in code in the test body. This improves the readability and ability to manage the tests.

Once this is merged, I have some other improvements that build on top of it to modernize this addon and optionally enable caching. This is part of build performance work funded by @PrecisionNutrition.

[lukemelia]

Thanks. I've rebased on revised #77