TF_EXPECT_OK(ReadCache(&cache, "b", 0, 1, &out));
  EXPECT_EQ(calls, 3);
  // Advance the fake time to `now` + 3, at which point "b" becomes stale.
  env->SetNowSeconds(now + 3);
  // Wait for the pruner to remove "b".
  start = Env::Default()->NowSeconds();
  do {
    Env::Default()->SleepForMicroseconds(100000);
  } while (cache.CacheSize() == 8 && Env::Default()->NowSeconds() - start < 3);