If you are using Symfony2 Standard Edition
Your config_dev.yml looks something like this for a monologue out of the box:
 # config_dev.yml monolog: handlers: main: type: fingers_crossed action_level: error handler: nested nested: type: stream path: %kernel.logs_dir%/%kernel.environment%.log level: debug 
As you can see, this defines the main and nested handlers, where nested used only because main refers to it.
config_dev.yml imported from config_test.yml , so if you want to override the configuration for the test environment, you need to override the main handler in config_test.yml :
 # config_text.yml monolog: handlers: main: type: test 
This will stop the monologue from creating the log file.