Fallback to w3c-test.org

So at the moment the serve.py script tries to do some cleverness and 
fall back to w3c-test.org if the local subdomains don't resolve e.g. it 
will try to use www.w3c-test.org in place of www.web-platform.test if 
the latter doesn't resolve.

Unfortunately this system is totally broken because the ports on the 
local machine and on w3c-test.org don't match at all. To deal with this, 
it would be necessary to assign each domain / subdomain a port either 
from loclhost or from the well-kown ports on the fallback server. This 
would impact the way that substitutions are used e.g.

{{domains[www]}}:{{ports[http][0]}}

wouldn't make any sense because the port would depend on the domain. 
Since tests are using various combinations of host/domain/port, fixing 
this would require a great deal of effort not just to design a 
fallback-aware substitution mechanism, but also to convert all tests to 
use it.

I don't think I have time to do this work (that is; I think there are 
other things I could be doing that are higher value). Does anyone else 
think that the the fallback to w3c-test.org is valuable enough that they 
are prepared to put in the effort to unbreak it? If not I suggest we 
remove this feature and simply require people to set up their 
environment correctly.