Concurrent rendering for ReactART-test

[jackpope]

Summary

We need to unblock flipping the default for RTR to be concurrent rendering. Update ReactART-test to use unstable_isConcurrent in place.

How did you test this change?

yarn test packages/react-art/src/__tests__/ReactART-test.js

[react-sizebot]

Comparing: 0aab065eb3250a9714a62dc05587cbb571da7f71...525d5fd1355436d92cb0f5717727d6acd732df7e

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	=	176.83 kB	176.83 kB	=	54.91 kB	54.91 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	173.55 kB	173.55 kB	=	54.11 kB	54.11 kB
facebook-www/ReactDOM-prod.classic.js	=	594.27 kB	594.27 kB	=	104.44 kB	104.44 kB
facebook-www/ReactDOM-prod.modern.js	=	577.53 kB	577.53 kB	=	101.47 kB	101.47 kB
test_utils/ReactAllWarnings.js	Deleted	66.63 kB	0.00 kB	Deleted	16.28 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	66.63 kB	0.00 kB	Deleted	16.28 kB	0.00 kB

Generated by :no_entry_sign: dangerJS against 525d5fd1355436d92cb0f5717727d6acd732df7e

[eps1lon]

Seems like various www tests are failing with these changes