A test base on the external runner feature depends on both the
external runner and the test URL. These two are always combined to
generate the complete command that will be executed by Avocado.

But, the Avocado command line application does not require URLs to be
given on the command line, because these may come from other sources
on loaders other than the basic FileLoader.

So, let's also explicitly require a URL to be given to to the External
Runner loader to return a test factory (class and parameters).
Signed-off-by: NCleber Rosa <crosa@redhat.com>