Review of literature for API mocking and testing

[noamross]

A pretty frequent topic of conversation in rOpenSci circle is approaches to testing packages that call Web APIs. I think this topic could use a "review of literature" - a systematic look at the different packages for this in the ecosystem, how their interfaces differ, the advantages and disadvantages of each, missing functionalities to be developed, and best practices/design patterns for testing packages.

Outputs for this might just be a blog post or vignette, or a series of issues and PRs across the package ecosystem.

Notably @sckott is nearing release of vcr, it could host the vignette and this review might be good for making sure it covers the use-cases that we explore.