Deflake DBBloomFilterTest.OptimizeFiltersForHits (#11383)

Summary:
In CircleCI build-linux-arm-test-full job (https://app.circleci.com/pipelines/github/facebook/rocksdb/26462/workflows/a9d39d2c-c970-4b0f-9c10-7743beb9771b/jobs/591722), this test exhibited the following flaky failure:

```
db/db_bloom_filter_test.cc:2506: Failure
Expected: (TestGetTickerCount(options, BLOOM_FILTER_USEFUL)) > (65000 * 2), actual: 120558 vs 130000
```

I ssh'd to an instance and observed it cuts memtables at slightly different points across runs. Logging in `ConcurrentArena` pointed to `try_lock()` returning false at different points across runs.

This PR changes the approach to allow a fixed number of keys per memtable flush. I verified the bloom filter useful count is deterministic now even on the CircleCI ARM instance.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11383

Reviewed By: cbi42

Differential Revision: D45036829

Pulled By: ajkr

fbshipit-source-id: b602dacb63955f1af09bf0ed409cde0552805a08

db/db_bloom_filter_test.cc

@@ -2437,9 +2437,11 @@ TEST_F(DBBloomFilterTest, PrefixScan) {
 }
 
 TEST_F(DBBloomFilterTest, OptimizeFiltersForHits) {
+  const int kNumKeysPerFlush = 1000;
+
   Options options = CurrentOptions();
-  options.write_buffer_size = 64 * 1024;
-  options.arena_block_size = 4 * 1024;
+  options.memtable_factory.reset(
+      test::NewSpecialSkipListFactory(kNumKeysPerFlush));
   options.target_file_size_base = 64 * 1024;
   options.level0_file_num_compaction_trigger = 2;
   options.level0_slowdown_writes_trigger = 2;
@@ -2475,8 +2477,13 @@ TEST_F(DBBloomFilterTest, OptimizeFiltersForHits) {
   int num_inserted = 0;
   for (int key : keys) {
     ASSERT_OK(Put(1, Key(key), "val"));
-    if (++num_inserted % 1000 == 0) {
-      ASSERT_OK(dbfull()->TEST_WaitForFlushMemTable());
+    num_inserted++;
+    // The write after each `kNumKeysPerFlush` keys triggers a flush. Always
+    // wait for that flush and any follow-on compactions for deterministic LSM
+    // shape.
+    if (num_inserted > kNumKeysPerFlush &&
+        num_inserted % kNumKeysPerFlush == 1) {
+      ASSERT_OK(dbfull()->TEST_WaitForFlushMemTable(handles_[1]));
       ASSERT_OK(dbfull()->TEST_WaitForCompact());
     }
   }