Swap to sinon stub from homerolled fakeConfig

[ryanluker]

Description

Finished part 3 of #321 to use sinon instead of the manual fake. Along with removing the old exportAPI test helper. cc @mattseddon

Design

Originally I was thinking of using sinon mocks but after reading though the documentation [1] it appears that these are more correctly called stubs in sinon land, heh [2].

I also turfed the exportAPI helpers for the extension tests and adopted the sinon spy instead [3], pretty happy with the results!

Work

• [x] use sinon stub for fakeConfig
• [x] refactor fakeConfig name to stub
• [x] remove exportAPI concept and use sinon spies instead

Research

1. https://sinonjs.org/releases/v11.1.1/mocks/#when-to-not-use-mocks
2. https://sinonjs.org/releases/v11.1.1/stubs/#when-to-use-stubs
3. https://sinonjs.org/releases/v11.1.1/spies/#using-a-spy-to-wrap-all-object-method
4. https://stackoverflow.com/a/38152098

[ryanluker]

@mattseddon thanks for the feedback!