[Bug] BlockFileCacheTest flaky: background thread interference and async open timeout

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

master

### What's Wrong?

`BlockFileCacheTest` has two types of flaky failures:

---

**Type 1: Background thread interference — `ttl_modify` failure**

The `test_file_cache()` helper creates a `BlockFileCache` that starts background threads. These threads asynchronously modify cache state between test assertions, causing `file_block->state()` to return EMPTY instead of SKIP_CACHE:

```
[ RUN ] BlockFileCacheTest.ttl_modify
be/test/io/cache/block_file_cache_test.cpp:447: Failure
Expected: file_block->state() == io::FileBlock::State::SKIP_CACHE
Actual: EMPTY == SKIP_CACHE
```

Root cause: the background `evict_in_advance` thread evicts releasable DOWNLOADED blocks, freeing space so that `try_reserve()` unexpectedly succeeds, keeping the state as EMPTY instead of transitioning to SKIP_CACHE.

---

**Type 2: Background thread interference — `io_error` failure**

Same root cause as Type 1, but manifests as incorrect block count because EMPTY blocks are not removed due to `use_count>2` from background thread references:

```
[ RUN ] BlockFileCacheTest.io_error
be/test/io/cache/block_file_cache_test.cpp:530: Failure
Expected: mgr.get_file_blocks_num(key) == 9
Actual: 10 == 9
```

Root cause: when a `FileBlocksHolder` destructor tries to remove EMPTY blocks (`use_count()==2`), the background thread or another holder still holds a reference (`use_count()>2`), preventing removal and leaving the block in the queue.

---

**Type 3: Insufficient async open timeout — `evict_privilege_order_for_ttl` failure**

`initialize()` starts a background disk I/O loading thread that sets `_async_open_done=true` only on completion. Tests wait only 100ms (100 iterations × 1ms), which is insufficient under high CPU load:

```
[ RUN ] BlockFileCacheTest.evict_privilege_order_for_ttl
be/test/io/cache/block_file_cache_test.cpp:6980: Failure
Expected: cache.get_or_set(key1, offset, 100000, context1) to succeed
(async open not completed, cache not ready)
```

Root cause: the 100ms total timeout is too short when the system is under load. The background loading thread needs more time to complete disk I/O and set `_async_open_done=true`.

---

### What You Expected?

Tests should be deterministic and not affected by background threads or timing issues.

### How to Reproduce?

Run `BlockFileCacheTest` repeatedly under CPU load. The flaky failures appear intermittently.

### Anything Else?

All three are test defects, not business code defects. The fix:
1. For Type 1 & 2: Save and restore config values, set all background thread intervals to 10000000ms during `test_file_cache` and `test_file_cache_memory_storage` to prevent background thread interference.
2. For Type 3: Extract `wait_for_async_open()` helper with 1000 iterations × 10ms (10s total timeout), replace all 91 inline wait loops. The 10ms sleep interval also avoids exacerbating CPU pressure under load compared to the original 1ms interval.

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!