StressTest - Move some stderr messages to stdout (facebook#12304)

Summary:
Moving some of the messages that we print out in `stderr` to `stdout` to make `stderr` more strictly related to errors.

Pull Request resolved: facebook#12304

Test Plan:
```
$> python3 tools/db_crashtest.py blackbox --simple --max_key=25000000 --write_buffer_size=4194304
```

**Before**
```
...
stderr:
WARNING: prefix_size is non-zero but memtablerep != prefix_hash
Error : jewoongh injected test error This is not a real failure.
Verification failed :(
```

**After**

No longer seeing the `WARNING: prefix_size is non-zero but memtablerep != prefix_hash` message in stderr, but still appears in stdout.

```
WARNING: prefix_size is non-zero but memtablerep != prefix_hash
Integrated BlobDB: blob files enabled 0, min blob size 0, blob file size 268435456, blob compression type NoCompression, blob GC enabled 0, cutoff 0.250000, force threshold 1.000000, blob compaction readahead size 0, blob file starting level 0
Integrated BlobDB: blob cache disabled
DB path: [/tmp/jewoongh/rocksdb_crashtest_blackboxzztp281q]
(Re-)verified 0 unique IDs
2024/01/29-11:57:51  Initializing worker threads
Crash-recovery verification passed :)
2024/01/29-11:57:58  Starting database operations
2024/01/29-11:57:58  Starting verification
Stress Test : 245.167 micros/op 10221 ops/sec
            : Wrote 0.00 MB (0.10 MB/sec) (16% of 6 ops)
            : Wrote 1 times
            : Deleted 0 times
            : Single deleted 0 times
            : 4 read and 0 found the key
            : Prefix scanned 0 times
            : Iterator size sum is 0
            : Iterated 3 times
            : Deleted 0 key-ranges
            : Range deletions covered 0 keys
            : Got errors 0 times
            : 0 CompactFiles() succeed
            : 0 CompactFiles() did not succeed

stderr:
Error : jewoongh injected test error This is not a real failure.
Error : jewoongh injected test error This is not a real failure.
Error : jewoongh injected test error This is not a real failure.
Verification failed :(
```

Reviewed By: akankshamahajan15

Differential Revision: D53193587

Pulled By: jaykorean

fbshipit-source-id: 40d59f4c993c5ce043c571a207ccc9b74a0180c6

db_stress_tool/db_stress_common.cc

@@ -200,7 +200,7 @@ void CompressedCacheSetCapacityThread(void* v) {
         // Lower by upto 50% of usable block cache capacity
         adjustment = (adjustment * thread->rand.Uniform(50)) / 100;
         block_cache->SetCapacity(capacity - adjustment);
-        fprintf(stderr, "New cache capacity = %lu\n",
+        fprintf(stdout, "New cache capacity = %lu\n",
                 block_cache->GetCapacity());
         db_stress_env->SleepForMicroseconds(10 * 1000 * 1000);
         block_cache->SetCapacity(capacity);
@@ -210,7 +210,7 @@ void CompressedCacheSetCapacityThread(void* v) {
             (double)thread->rand.Uniform(
                 FLAGS_compressed_secondary_cache_ratio * 100) /
             100;
-        fprintf(stderr, "New comp cache ratio = %f\n", new_comp_cache_ratio);
+        fprintf(stdout, "New comp cache ratio = %f\n", new_comp_cache_ratio);
 
         s = UpdateTieredCache(block_cache, /*capacity*/ -1,
                               new_comp_cache_ratio);

db_stress_tool/db_stress_test_base.cc

@@ -2701,7 +2701,7 @@ void StressTest::Open(SharedState* shared, bool reopen) {
     exit(1);
   }
   if (FLAGS_prefix_size != 0 && FLAGS_rep_factory != kHashSkipList) {
-    fprintf(stderr,
+    fprintf(stdout,
             "WARNING: prefix_size is non-zero but "
             "memtablerep != prefix_hash\n");
   }