fix up various test cases

A few test cases were relying on being able to mount non-client servlets on the
test resource. it's better to give them their own Resources.

tests/server.py

@@ -216,8 +216,9 @@ def make_request(
         and not path.startswith(b"/_matrix")
         and not path.startswith(b"/_synapse")
     ):
+        if path.startswith(b"/"):
+            path = path[1:]
         path = b"/_matrix/client/r0/" + path
-        path = path.replace(b"//", b"/")
 
     if not path.startswith(b"/"):
         path = b"/" + path