cover single level universal in crash test (#6818)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6818

Test Plan:
fast whitebox test and verify there are some single-level universal and
some multi-level universal runs.

```
$ python ./tools/db_crashtest.py whitebox --simple -max_key=1000000 -value_size_mult=33 -write_buffer_size=524288 -target_file_size_base=524288 -max_bytes_for_level_base=2097152 --duration=120 --interval=10 --ops_per_thread=1000 --random_kill_odd=887
```

Reviewed By: riversand963

Differential Revision: D21432138

Pulled By: ajkr

fbshipit-source-id: 2fc5ba9f3dfa49bb11e81da7dd00a17b476e64d7

db_stress_tool/db_stress_common.h

@@ -112,6 +112,7 @@ DECLARE_bool(memtable_whole_key_filtering);
 DECLARE_int32(open_files);
 DECLARE_int64(compressed_cache_size);
 DECLARE_int32(compaction_style);
+DECLARE_int32(num_levels);
 DECLARE_int32(level0_file_num_compaction_trigger);
 DECLARE_int32(level0_slowdown_writes_trigger);
 DECLARE_int32(level0_stop_writes_trigger);

db_stress_tool/db_stress_gflags.cc

@@ -186,6 +186,9 @@ DEFINE_int64(compressed_cache_size, -1,
 DEFINE_int32(compaction_style, ROCKSDB_NAMESPACE::Options().compaction_style,
              "");
 
+DEFINE_int32(num_levels, ROCKSDB_NAMESPACE::Options().num_levels,
+             "Number of levels in the DB");
+
 DEFINE_int32(level0_file_num_compaction_trigger,
              ROCKSDB_NAMESPACE::Options().level0_file_num_compaction_trigger,
              "Level0 compaction start trigger");

tools/db_crashtest.py

@@ -422,6 +422,12 @@ def whitebox_crash_main(args, unknown_args):
                 "ops_per_thread": cmd_params['ops_per_thread'],
                 "compaction_style": 1,
             }
+            # Single level universal has a lot of special logic. Ensure we cover
+            # it sometimes.
+            if random.randint(0, 1) == 1:
+                additional_opts.update({
+                    "num_levels": 1,
+                })
         elif check_mode == 2:
             # normal run with FIFO compaction mode
             # ops_per_thread is divided by 5 because FIFO compaction