DEV: Move discourse-chat to the core repo. (#18776)

As part of this move, we are also renaming `discourse-chat` to `chat`.

plugins/chat/test/javascripts/helpers/mock-presence-channel.js

@@ -0,0 +1,15 @@
+import EmberObject from "@ember/object";
+
+export default class MockPresenceChannel extends EmberObject {
+  users = [];
+  name = null;
+  subscribed = false;
+
+  async unsubscribe() {
+    this.set("subscribed", false);
+  }
+
+  async subscribe() {
+    this.set("subscribed", true);
+  }
+}