Add ProxyTestCase

This depends on FriendsOfSymfony/FosHttpCache#103. 

It adds the following config:

```yaml
fos_http_cache:
    proxy_server:
        varnish:
            binary: /usr/sbin/varnishd
            port: 8080
            config_file: /etc/varnish/default.vcl
```
Other setters may be added here, depending on the config that we allow.