crash_test: use large max_manifest_file_size most of the time. (#6034)

Summary:
Right now, crash_test always uses 16KB max_manifest_file_size value. It is good to cover logic of manifest file switch. However, information stored in manifest files might be useful in debugging failures. Switch to only use small manifest file size in 1/15 of the time.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6034

Test Plan: Observe command generated by db_crash_test.py multiple times and see the --max_manifest_file_size value distribution.

Differential Revision: D18513824

fbshipit-source-id: 7b3ae6dbe521a0918df41064e3fa5ecbf2466e04

tools/db_crashtest.py

@@ -72,6 +72,10 @@ default_params = {
     "periodic_compaction_seconds" :
         lambda: random.choice([0, 0, 1, 2, 10, 100, 1000]),
     "compaction_ttl" : lambda: random.choice([0, 0, 1, 2, 10, 100, 1000]),
+    # Test small max_manifest_file_size in a smaller chance, as most of the
+    # time we wnat manifest history to be preserved to help debug
+    "max_manifest_file_size" : lambda : random.choice(
+        [t * 16384 if t < 3 else 1024 * 1024 * 1024 for t in range(1,30)])
 }
 
 _TEST_DIR_ENV_VAR = 'TEST_TMPDIR'