Add "fetch-ponyfill" polyfill to enzyme test stuite configuration

[nwalberts]

The enzyme test suite needs a polyfill for some standard JS objects (fetch, Request, Response, etc) in order to use the fetch-mock library.

• add fetch-ponyfill to package.json file
• import http objects into test helper file and assign to global test environment

This is the doc that was used for Bos-23 to get them up to speed on their projects (see the bottom)
https://gist.github.com/nwalberts/2e8d050d6037d0a1b549e9467bb077c9

[dpickett]

@nwalberts can you confirm these are still actionable? I'm thinking that even with the jest migration, we will need to enact these changes.

Thanks!