crash_test to cover index_type kBinarySearchWithFirstKey (#6721)

Summary:
Recently index_type kBinarySearchWithFirstKey is improved so that the API guarantee is exactly the same as other types and it is ready for wide production. We should cover it in crash tst.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6721

Test Plan: Run crash_test

Reviewed By: anand1976

Differential Revision: D21099781

fbshipit-source-id: fda91eba831d9eacbb140c703e9768bb1701f935

tools/db_crashtest.py

@@ -59,7 +59,7 @@ default_params = {
     "get_sorted_wal_files_one_in": 0,
     "get_current_wal_file_one_in": 0,
     # Temporarily disable hash index
-    "index_type": lambda: random.choice([0,2]),
+    "index_type": lambda: random.choice([0, 0, 0, 2, 2, 3]),
     "max_background_compactions": 20,
     "max_bytes_for_level_base": 10485760,
     "max_key": 100000000,