fix: ensure build-less browser tests work with swivel-js v3

[alexanderwende]

the fetch module in internal uses cross-fetch, which shims the fetch method on node environments which don't yet support it

the problem occurs when running build-less es6 modules in browsers (during tests) when cross-fetch resolves itself to the /dist/browser-ponyfill.js which doesn't export the window's Request class:

https://github.com/Swivel-Finance/swivel-js/blob/v3/src/internal/fetch/index.ts#L55