Do not treat a prefix of / as a real URL prefix.

tests/wpt/harness/wptrunner/executors/webdriver.py

@@ -177,7 +177,7 @@ def __init__(self, host, port, url_prefix="", port_timeout=60):
         self.host = host
         self.port = port
         self.port_timeout = port_timeout
-        if url_prefix == "":
+        if url_prefix == "" or url_prefix == "/":
             self.path_prefix = "/"
         else:
             self.path_prefix = "/%s/" % url_prefix.strip("/")