test: add delay for flaky focus test

Author: huozhi

test/use-swr-focus.test.tsx

@@ -198,6 +198,7 @@ describe('useSWR - focus', () => {
     // trigger revalidation
     await focusWindow()
     await waitForNextTick()
+    await act(() => sleep(50))
     await screen.findByText('data: 5')
   })