Wait for process completion for sflow script (#15809)

The process.terminate() is sending SIGTERM to sflow but there is a missing process.wait() to wait for process to exit. So, likelly the file content was buffered and not fully written by the time we open the file for reading

Author: weiguo-nvidia

ansible/roles/test/files/ptftests/py3/sflow_test.py

@@ -119,6 +119,7 @@ def read_data(self, collector, event, sflow_port=['6343']):
                 threading.current_thread().getName(), event_is_set))
 
         process.terminate()
+        process.wait()
         f.close()
         with open(outfile, 'r') as sflow_data:
             for line in sflow_data: