Expand stress test coverage for min_write_buffer_number_to_merge (#10785)

Summary:
As title.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10785

Test Plan: CI

Reviewed By: ltamasi

Differential Revision: D40162583

Pulled By: ltamasi

fbshipit-source-id: 4e01f9b682f397130e286cf5d82190b7973fa3c1

tools/db_crashtest.py

@@ -201,6 +201,7 @@ default_params = {
     "initial_auto_readahead_size": lambda: random.choice([0, 16384, 524288]),
     "max_auto_readahead_size": lambda: random.choice([0, 16384, 524288]),
     "num_file_reads_for_auto_readahead": lambda: random.choice([0, 1, 2]),
+    "min_write_buffer_number_to_merge": lambda: random.choice([1, 2]),
 }
 
 _TEST_DIR_ENV_VAR = "TEST_TMPDIR"