Fix an intermittent test failure

Logging goes via an asynchronous queue, so the log message not have
reached Python by the time we check for it. For now just work around it
by adding a sleep.

spead2/test/test_recv.py

@@ -625,6 +625,8 @@ def test_disallow_unsized_heaps(self):
         if assert_logs is not None:
             with assert_logs('spead2', 'INFO') as cm:
                 heaps = self.data_to_heaps(packet, allow_unsized_heaps=False)
+                # Logging is asynchronous, so we have to give it a bit of time
+                time.sleep(0.1)
             assert_equal(cm.output, ['INFO:spead2:packet rejected because it has no HEAP_LEN'])
         else:
             # Python 2 fallback