test: fix flaky tests for focus and location mutation

[koba04]

Fixes #940

I've inspected why the focus tests are flaky.
I've found that some tests didn't wait for a deduping interval, which means that a mutation in the next test might be deduped and failed.
To fix this, I've added await act(() => sleep(1)) before the next mutation. But the intention of the code is very unclear, so I've separated it as another function called waitForDedupingInterval.

In addition, many of the current tests depend on a real timer, which makes tests flaky, so I've refactored to avoid depending on a real timer as much possible as I can by using screen.findByText.

I've also noticed that a test in use-swr-local-mutation.test.tsx is flaky, so I've fixed it too.

I think there are more flaky tests in SWR, so I'll fix them so that tests are more reliable. #932 and #933 are PRs for it.

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[shuding]

Looks great, thank you!

[shuding]

It might be a nitpick, but I think the name waitForNextTick will be better than waitForDedupingInterval. Because it might not be caused by the deduping here, the state broadcasting is also asynchronous (check #735).

(But it’s just a suggestion and will not block the PR, so I’ll merge this one for now 👍. Thanks!)

[koba04]

Thank you! waitForNextTick makes sense for me, so I'll create a PR for that👍