BrowserKit simulates the behavior of a web browser.
The component only provide an abstract client and does not provide any "default" backend for the HTTP layer.
For a simple implementation of a browser based on an HTTP layer, have a look at Goutte.
For an implementation based on HttpKernelInterface, have a look at the Client provided by the HttpKernel component.
You can run the unit tests with the following command:
$ cd path/to/Symfony/Component/BrowserKit/
$ composer.phar install
$ phpunit