[#276] Context manager for parralel func

Signed-off-by: Dmitriy Zayakin <d.zayakin@yadro.com>

src/frostfs_testlib/testing/parallel.py

@@ -1,10 +1,22 @@
 import itertools
 from concurrent.futures import Future, ThreadPoolExecutor
+from contextlib import contextmanager
 from typing import Callable, Collection, Optional, Union
 
 MAX_WORKERS = 50
 
 
+@contextmanager
+def parallel_workers_limit(workers_count: int):
+    global MAX_WORKERS
+    original_value = MAX_WORKERS
+    MAX_WORKERS = workers_count
+    try:
+        yield
+    finally:
+        MAX_WORKERS = original_value
+
+
 def parallel(
     fn: Union[Callable, list[Callable]],
     parallel_items: Optional[Collection] = None,