[ci] Separate DevTools test-build into dedicated job with fewer shards

[poteto]

DevTools has ~45 test files which don't distribute well across 10 shards,
causing shard 3 to run 2x slower than others (104s vs ~50s). This moves
DevTools build tests to a separate job with 3 shards for better load balancing.

[react-sizebot]

Comparing: 65eec42...e27e1c9

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	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	607.60 kB	607.60 kB	=	107.53 kB	107.54 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	666.83 kB	666.83 kB	=	117.42 kB	117.42 kB
facebook-www/ReactDOM-prod.classic.js	=	692.91 kB	692.91 kB	=	121.92 kB	121.92 kB
facebook-www/ReactDOM-prod.modern.js	=	683.34 kB	683.34 kB	=	120.31 kB	120.31 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against e27e1c9