[FIX] conftest: assert tunnel response is a proper URL

Since the tunnel value is now a path to a script to run (and get the
public facing address out of), it's pretty easy to give the path to
the wrong program e.g. `--tunnel ngrok` will run ngrok itself, which
is going to yield an error message and do nothing, and then break when
we try to use the nonsensical result (e.g. an empty string or an error
message) as a URL.

So assert that the result does parse as a URL-ish with at least a
scheme and netloc.

conftest.py

@@ -73,6 +73,7 @@ import xmlrpc.client
 from contextlib import closing
 from dataclasses import dataclass
 from typing import Optional
+from urllib.parse import urlsplit
 
 import pytest
 import requests
@@ -236,7 +237,10 @@ def tunnel(pytestconfig: pytest.Config, port: int):
         ) as p:
             # read() blocks forever and I don't know why, read things about the
             # write end of the stdout pipe still being open here?
-            yield p.stdout.readline().strip()
+            result = p.stdout.readline().strip()
+            url = urlsplit(result)
+            assert url.scheme and url.netloc
+            yield url.geturl()
             p.terminate()
             p.wait(30)
     else: