Add ref callback test for cleanup fn vs null call

[jackpope]

Used this test scenario to clarify how callback refs work when detached based on the availability of a cleanup function to update documentation in reactjs/react.dev#6770

Checking it in for additional test coverage and test-based documentation

[noahlemen]

would it be worth calling root.unmount() and making assertions on the behavior around that as well?

[noahlemen]

cleanUp2 doesn't seem to be used anywhere?

[react-sizebot]

Comparing: 699d03c...2ab758f

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	=	497.71 kB	497.71 kB	=	88.93 kB	88.93 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	504.00 kB	504.00 kB	=	89.95 kB	89.95 kB
facebook-www/ReactDOM-prod.classic.js	=	591.14 kB	591.14 kB	=	103.91 kB	103.91 kB
facebook-www/ReactDOM-prod.modern.js	=	566.95 kB	566.95 kB	=	100.12 kB	100.12 kB
test_utils/ReactAllWarnings.js	Deleted	64.80 kB	0.00 kB	Deleted	16.14 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	64.80 kB	0.00 kB	Deleted	16.14 kB	0.00 kB

Generated by 🚫 dangerJS against 2ab758f

[noahlemen]

oh... i see there's already another test covering what i suggested.

thanks!