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.
Can there ever be more than one listening socket in this test? I was expecting something more like `assert.ok(debuggerServerSockets > 0)`.