Allow base URL to be set in the class

We are running tests on a multisite install and we want to be able to run tests against both sites without the need to set up two phpunit.xml files and run the test separately. Since we are also running tests in parallel we can't manipulate the env setting at run time, so being able to set $this->baseUrl in the test class would allow us to add logic to determine which test should use which baseUrl.

This is basically the logic that is already available for $this->driver in getDriverInstance()