tests: fix flakiness in connection cleanup test (#487)

Author: plamut

tests/system.py

@@ -446,7 +446,11 @@ def test_subscriber_not_leaking_open_sockets(
  assert len(response.received_messages) == 3
 
  conn_count_end = len(current_process.connections())
- assert conn_count_end == conn_count_start
+
+ # To avoid flakiness, use <= in the assertion, since on rare occasions additional
+ # sockets are closed, causing the == assertion to fail.
+ # https://github.com/googleapis/python-pubsub/issues/483#issuecomment-910122086
+ assert conn_count_end <= conn_count_start
 
 
 def test_synchronous_pull_no_deadline_error_if_no_messages(