QueueReceiver unreliable test

[moggers87]

  ======================================================================
  FAIL: test_queue_receiver_sleep (tests.test_server.ServerTestCase)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/unittest/mock.py", line 1183, in patched
      return func(*args, **keywargs)
    File "/Users/runner/work/salmon/salmon/tests/test_server.py", line 303, in test_queue_receiver_sleep
      self.assertEqual(sleep_mock.call_count, 2)
  AssertionError: 3 != 2

I feel like I've seen this fail before. Maybe just checking the call count is greater than 0 would be enough to ensure correctness?