test: Use falsy values for test environment variable in flask_app

Signed-off-by: Ferenc Géczi <[email protected]>

Author: Ferenc-

tests/apps/flask_app/__init__.py

@@ -7,5 +7,5 @@
 
 app_thread = None
 
-if not os.environ('CASSANDRA_TEST') and app_thread is None:
+if not os.environ.get('CASSANDRA_TEST') and app_thread is None:
     app_thread = launch_background_thread(server.serve_forever, "Flask")