Update Suspense fuzz tests to use `act` (#26498)

This updates the Suspense fuzz tester to use `act` to recursively flush
timers instead of doing it manually.

This still isn't great because ideally the fuzz tester wouldn't fake
timers at all. It should resolve promises using a custom queue instead
of Jest's fake timer queue, like we've started doing in our other
Suspense tests (i.e. the `resolveText` pattern). That's because our
internal `act` API (not the public one, the one we use in our tests)
uses Jest's fake timer queue as a way to force Suspense fallbacks to
appear.

However I'm not interested in upgrading this test suite to a better
strategy right now because if I were writing a Suspense fuzzer today I
would probably use an entirely different approach. So this is just an
incremental improvement to make it slightly less decoupled to React
implementation details.