PYTHON-989 - Fix find_one_and_write_concern test when run with authentication.

test/test_collection.py

@@ -2003,6 +2003,9 @@ class TestCollection(IntegrationTest):
         # default WriteConcern.
         c_default = db.get_collection('test', write_concern=WriteConcern())
         results = listener.results
+        # Authenticate the client and throw out auth commands from the listener.
+        db.command('ismaster')
+        results.clear()
         try:
             if client_context.version.at_least(3, 1, 9, -1):
                 c_w0.find_and_modify(