dillon1000/react

Internal `act`: Flush timers at end of scope (#19788)

If there are any suspended fallbacks at the end of the `act` scope,
force them to display by running the pending timers (i.e. `setTimeout`).

The public implementation of `act` achieves the same behavior with an
extra check in the work loop (`shouldForceFlushFallbacks`). Since our
internal `act` needs to work in both development and production, without
additional runtime checks, we instead rely on Jest's mock timers.

This doesn't not affect refresh transitions, which are meant to delay
indefinitely, because in that case we exit the work loop without
posting a timer.