fix: silence unused-variable warning in TestServer.Plug.call/2

The opts arg is intentionally unused (we look up the server via
persistent_term instead). Underscore-prefix it so --warnings-as-errors
on our code (which CI now does, not --all-warnings on deps) doesn't
fail the build.

Discovered by the GitHub Actions run that actually exercises the
--warnings-as-errors flag end-to-end (local runs had this warning
swallowed somehow).

Author: gilbertwong96

test/support/test_server.ex

@@ -90,7 +90,7 @@ defmodule AdoCli.TestServer do
     def init(opts), do: opts
 
     @impl true
-    def call(conn, opts) do
+    def call(conn, _opts) do
       # The TestServer GenServer registers itself in :persistent_term
       # during init. We look it up here. This works because Bandit
       # calls call/2 in the same process tree (a worker of the TestServer