dillon1000/react

[Flight] Simulate fetch to third party in fixture (#33484)

This adds some I/O to go get the third party thing to test how it
overlaps.

With #33482, this is what it looks like. The await gets cut off when the
third party component starts rendering. I.e. after the latency to start.

<img width="735" alt="Screenshot 2025-06-08 at 5 42 46 PM"
src="https://github.com/user-attachments/assets/f68d9a84-05a1-4125-b3f0-8f3e4eaaa5c1"
/>

This doesn't fully simulate everything because it should actually also
simulate each chunk of the stream coming back too. We could wrap the
ReadableStream to simulate that. In that scenario, it would probably get
some awaits on the chunks at the end too.