[test] strict effect tests should fail

[rickhanlonii]

While doing https://github.com/facebook/react/pull/28102/files, I suspect these strict effect tests were not running. Creating a PR to run all the tests and hopefully some fail and I'm just missing something.

[react-sizebot]

Comparing: c7e735f...e6f5ee4

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	=	176.34 kB	176.34 kB	=	54.93 kB	54.93 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	178.44 kB	178.44 kB	=	55.57 kB	55.57 kB
facebook-www/ReactDOM-prod.classic.js	=	572.79 kB	572.79 kB	=	100.80 kB	100.80 kB
facebook-www/ReactDOM-prod.modern.js	=	556.55 kB	556.55 kB	=	97.88 kB	97.88 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

Generated by 🚫 dangerJS against e6f5ee4

[rickhanlonii]

Yeah, these tests haven't run for a year, since #25575.

This is fixed by #28102, which removes the flag and updates the tests to use StrictMode directly, instead of implicitly relying on this flag being a variant.

cc @kassens