tests: close coverage gap for race condition (#261)

Closes #260
diff --git a/tests/unit/test_bidi.py b/tests/unit/test_bidi.py
index b876d9a..15994ee 100644
--- a/tests/unit/test_bidi.py
+++ b/tests/unit/test_bidi.py
@@ -836,7 +836,7 @@
 
         # Wait for the consumer's thread to exit.
         while consumer.is_active:
-            pass
+            pass  # pragma: NO COVER (race condition)
 
         on_response.assert_not_called()
         bidi_rpc.recv.assert_called_once()