Bug 1033079 - Fix up DebuggerServer test after refactoring. r=janx

The `DebuggerServer` API is changing in bug 1033079, and after this fix lands, it won't have `_listener` anymore. This updates the test to use the new API.
No, not in this test. If we also turn on WiFi debugging (once that lands) at the same time, it would then be 2. I think this should be fine as-is.