[DevTool] fix build-for-devtools script

[mondaychen]

Summary

Add back react-dom/test-utils to yarn build-for-devtools as it's still used by tests such as yarn test --build --project=devtools -r=experimental

How did you test this change?

yarn build-for-devtools
yarn test --build --project=devtools -r=experimental

[sizebot]

Comparing: 56389e8...625d157

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	=	131.64 kB	131.64 kB	=	42.30 kB	42.30 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	136.92 kB	136.92 kB	=	43.89 kB	43.89 kB
facebook-www/ReactDOM-prod.classic.js	=	440.83 kB	440.83 kB	=	80.64 kB	80.64 kB
facebook-www/ReactDOM-prod.modern.js	=	426.13 kB	426.13 kB	=	78.45 kB	78.45 kB
facebook-www/ReactDOMForked-prod.classic.js	=	441.61 kB	441.61 kB	=	80.87 kB	80.87 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 625d157