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/utils/datetime_utils.py

@@ -10,6 +10,9 @@ def parse_time(value: str) -> int:
     Returns:
         Number of seconds in the parsed time interval.
     """
+    if value is None:
+        return 0
+
     value = value.lower()
 
     for suffix in ["s", "sec"]: