[WIP] Gate legacy render tests in ReactDOMSingletonComponents to www

[rickhanlonii]

WIP

TODO: Finish this PR to gate these to www once they fail elsewhere.

[rickhanlonii]

I can't land this yet, because in order to gate, they need to fail in the other environments.

Rebase and land this closer to release.

[react-sizebot]

Comparing: 1d5667a1273386f84e416059af7b6aba069e068e...64bd0b97bdfb92a4a364696b01f85f299091d481

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.min.js	=	175.90 kB	175.90 kB	=	54.76 kB	54.75 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	177.97 kB	177.97 kB	=	55.39 kB	55.39 kB
facebook-www/ReactDOM-prod.classic.js	=	570.21 kB	570.21 kB	=	100.35 kB	100.35 kB
facebook-www/ReactDOM-prod.modern.js	=	554.06 kB	554.06 kB	=	97.43 kB	97.43 kB
test_utils/ReactAllWarnings.js	Deleted	67.55 kB	0.00 kB	Deleted	16.54 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	67.55 kB	0.00 kB	Deleted	16.54 kB	0.00 kB

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