Allow longer duration in test_188_concurrency (#969)

Author: talltree-eng

tests/issues/test_188_concurrency.py

@@ -35,7 +35,7 @@ async def slow_resource():
         end_time = anyio.current_time()
 
         duration = end_time - start_time
-        assert duration < 6 * _sleep_time_seconds
+        assert duration < 10 * _sleep_time_seconds
         print(duration)