Deflake blob caching tests (#10636)

Summary:
Example failure:

```
db/blob/db_blob_basic_test.cc:226: Failure
Expected equality of these values:
  i
    Which is: 1
  num_blobs
    Which is: 5
```

I can't repro locally, but it looks like the 2KB cache is too small to guarantee no eviction happens between loading all the data into cache and reading from `kBlockCacheTier`. This 2KB setting appears to have come from a test where the cached entries are pinned, where it makes sense to have a small setting. However, such a small setting makes less sense when the blocks are evictable but must remain cached per the test's expectation. This PR increases the capacity setting to 2MB for those cases.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10636

Reviewed By: cbi42

Differential Revision: D39250976

Pulled By: ajkr

fbshipit-source-id: 769309f9a19cfac20b67b927805c8df5c1d2d1f5

db/blob/db_blob_basic_test.cc

@@ -54,7 +54,7 @@ TEST_F(DBBlobBasicTest, GetBlobFromCache) {
   Options options = GetDefaultOptions();
 
   LRUCacheOptions co;
-  co.capacity = 2048;
+  co.capacity = 2 << 20;  // 2MB
   co.num_shard_bits = 2;
   co.metadata_charge_policy = kDontChargeCacheMetadata;
   auto backing_cache = NewLRUCache(co);
@@ -122,7 +122,7 @@ TEST_F(DBBlobBasicTest, IterateBlobsFromCache) {
   Options options = GetDefaultOptions();
 
   LRUCacheOptions co;
-  co.capacity = 2048;
+  co.capacity = 2 << 20;  // 2MB
   co.num_shard_bits = 2;
   co.metadata_charge_policy = kDontChargeCacheMetadata;
   auto backing_cache = NewLRUCache(co);
@@ -453,7 +453,7 @@ TEST_F(DBBlobBasicTest, MultiGetBlobsFromCache) {
   Options options = GetDefaultOptions();
 
   LRUCacheOptions co;
-  co.capacity = 2048;
+  co.capacity = 2 << 20;  // 2MB
   co.num_shard_bits = 2;
   co.metadata_charge_policy = kDontChargeCacheMetadata;
   auto backing_cache = NewLRUCache(co);