The sink is what we need to reset most of the times.
This is an attempt to make the test suite faster and more stable
on macOS.

The tests all spin up their own service instance, this should be fine.

We're the only ones using it

For an unknown reason the upload tests fail on Node 18 on Windows
with an ECONNRESET from node-fetch. Debugging hasn't revealed any
probable cause.

- The fixture readable stream works as expected on Node 18 on Windows
- Other requests to the server work as expected

Refactoring to use native fetch means we lose the option to override
the origin hostname, which we use to test server validation.