test: Refactor util tests to use local webserver

tests/test_util.py

@@ -1,5 +1,5 @@
 from archivebox import util
 
 def test_download_url_downloads_content():
-    text = util.download_url("https://example.com")
+    text = util.download_url("http://localhost:8080/static/example.com.html")
     assert "Example Domain" in text