1. Increase wait time for k6 teardown after stop signal 2. Remove duplicated code

Signed-off-by: Andrey Berezin <a.berezin@yadro.com>

src/frostfs_testlib/testing/test_control.py

@@ -123,6 +123,7 @@ def wait_for_success(
     interval: int = 1,
     expected_result: Any = None,
     fail_testcase: bool = False,
+    fail_message: str = "",
 ):
     """
     Decorator to wait for some conditions/functions to pass successfully.
@@ -141,7 +142,7 @@ def wait_for_success(
                 try:
                     actual_result = func(*a, **kw)
                     if expected_result is not None:
-                        assert expected_result == actual_result
+                        assert expected_result == actual_result, fail_message
                     return actual_result
                 except Exception as ex:
                     logger.debug(ex)