Fix `compression_zstd_max_train_bytes` coverage in stress test (#4957)

Summary:
Previously `finalize_and_sanitize` function was always zeroing out `compression_zstd_max_train_bytes`. It was only supposed to do that when non-ZSTD compression was used. But since `--compression_type` was an unknown argument (i.e., one that `db_crashtest.py` does not recognize and blindly forwards to `db_stress`), `finalize_and_sanitize` could not tell whether ZSTD was used. This PR fixes it simply by making `--compression_type` a known argument with snappy as default (same as `db_stress`).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4957

Differential Revision: D13994302

Pulled By: ajkr

fbshipit-source-id: 1b0baea7331397822830970d3698642eb7a7df65

tools/db_crashtest.py

@@ -26,6 +26,7 @@ default_params = {
     "block_size": 16384,
     "cache_size": 1048576,
     "checkpoint_one_in": 1000000,
+    "compression_type": "snappy",
     "compression_max_dict_bytes": lambda: 16384 * random.randint(0, 1),
     "compression_zstd_max_train_bytes": lambda: 65536 * random.randint(0, 1),
     "clear_column_family_one_in": 0,