Convert more Suspense tests to use act (facebook#26602)

Many of our Suspense-related tests were written before the `act` API was
introduced, and use the lower level `waitFor` helpers instead. So they
are less resilient to changes in implementation details than they could
be.

This converts some of our test suite to use `act` in more places. I
found these while working on a PR to expand our fallback throttling
mechanism to include all renders that result from a promise resolving,
even if there are no more fallbacks in the tree. This isn't all the
affected tests, just some of them — I'll be sharding the changes across
multiple PRs.