Fix DevTools test target

[bvaughn]

This regressed in #21238 so that yarn test-build-devtools no longer ran.

[sizebot]

Comparing: 6b3d86a...69c66f0

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	=	122.68 kB	122.68 kB	=	39.38 kB	39.38 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	129.25 kB	129.25 kB	=	41.46 kB	41.46 kB
facebook-www/ReactDOM-prod.classic.js	=	412.18 kB	412.18 kB	=	76.23 kB	76.23 kB
facebook-www/ReactDOM-prod.modern.js	=	400.23 kB	400.23 kB	=	74.32 kB	74.32 kB
facebook-www/ReactDOMForked-prod.classic.js	=	412.18 kB	412.18 kB	=	76.23 kB	76.23 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 69c66f0

[bvaughn]

Going to merge this for now b'c I have to keep patching my local branches to run the DevTools test command :) Let me know if you'd like follow up changes.