Add failing test for Suspense layout semantics

[gaearon]

Failing test for #21676.

For some reason I couldn't repro it with noop renderer, which is used for all related tests like here.

[sizebot]

Comparing: 1a106bd...ce8d95b

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	=	126.89 kB	126.89 kB	=	40.69 kB	40.69 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	129.70 kB	129.70 kB	=	41.61 kB	41.61 kB
facebook-www/ReactDOM-prod.classic.js	=	406.00 kB	406.00 kB	=	75.07 kB	75.07 kB
facebook-www/ReactDOM-prod.modern.js	=	394.35 kB	394.35 kB	=	73.25 kB	73.25 kB
facebook-www/ReactDOMForked-prod.classic.js	=	406.00 kB	406.00 kB	=	75.07 kB	75.07 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against ce8d95b

[gaearon]

What I'm seeing debugging this is that after Suspense is re-shown, "c" does not get logged:

So this tells me that we don't apply LayoutMask to subtreeFlags for some reason though we should have. (maybe?)

[bvaughn]

Fix in #21694