uint64 now = Env::Default()->NowSeconds();
  env->SetNowSeconds(now);
  tf_gcs_filesystem::RamFileBlockCache cache(
      8, 32, 1 /* max staleness */, fetcher,
      [&env]() { return env->NowSeconds(); });
  // Read three blocks into the cache, and advance the timestamp by one second
  // with each read. Start with a block of "a" at the current timestamp `now`.
  TF_EXPECT_OK(ReadCache(&cache, "a", 0, 1, &out));