Use random port for application server
Especially in CI environments it would be very useful to be able to run several tests with gocept.selenium in parallel -- which requires that the app server not listen on the same, hard-coded port.
Opening port 0 makes the Kernel to open a (guaranteed available) random port, we just need to figure out how to get the actual port number, since we need to tell it to the browser.
Open a random port for the server process by default
When the environment variable `GOCEPT_SELENIUM_APP_PORT` is not set, a random
free port is bound. This allows parallel testing, for instance (re #11323).