CRA test environment no longer compatible with version 6.4.5 of react-router

[hovsater]

Describe the bug

For a detailed explanation of the problem, see remix-run/react-router#9722 (comment), which is the issue I originally opened.

CRA automatically injects fetch in the Jest test environment. Since fetch isn't spec-compliant (it does not expose a body property) it fail the check introduced by react-router, version 6.4.5, thus making react-router misbehave.

A suggestion was made by @brophdawg11 to use @remix-run/web-fetch instead, but this isn't necessarily easy to implement as there's no exposed entrypoints in CRA to inject the polyfill before fetch gets injected.

[hovsater]

Ping @gaearon, @ianschmitz, @mrmckeb, @matrush. Anyone really. I'd be happy to assist with help, code or whatever. Unless this is solved, CRA is officially incompatible with react-router as they've clearly stated they won't add code to support a fetch implementation that isn't spec compliant.