[Flight] Simulate fetch to third party in fixture

[sebmarkbage]

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.

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.

[react-sizebot]

Comparing: c0b5a0c...b2778a9

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.07 kB	530.07 kB	=	93.57 kB	93.57 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	651.16 kB	651.16 kB	=	114.69 kB	114.70 kB
facebook-www/ReactDOM-prod.classic.js	=	676.11 kB	676.11 kB	=	118.97 kB	118.97 kB
facebook-www/ReactDOM-prod.modern.js	=	666.39 kB	666.39 kB	=	117.36 kB	117.36 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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