dillon1000/react

Bugfix: Legacy Mode + DevTools "force fallback" (#19164)

DevTools has a feature to force a Suspense boundary to show a fallback.
This feature causes us to skip the first render pass (where we render
the primary children) and go straight to rendering the fallback.

There's a Legacy Mode-only codepath that failed to take this scenario
into account, instead assuming that whenever a fallback is being
rendered, it was preceded by an attempt to render the primary children.

SuspenseList can also cause us to skip the first pass, but the relevant
branch is Legacy Mode-only, and SuspenseList is not supported in
Legacy Mode.

Fixes a test that I had temporarily disabled when upstreaming the Lanes
implementation in #19108.