Firebase Stub refactoring

__mocks__/gatsby-plugin-firebase/auth/auth.js

@@ -0,0 +1,51 @@
+import { v4 as uuidv4 } from 'uuid';
+
+import { AuthConstants } from '../constants';
+
+class Auth {
+  static #instance = undefined;
+  #uuid = '';
+  #onAuthStateChangedObservers = [];
+
+  constructor() {
+    if (Auth.#instance) {
+      return Auth.#instance;
+    }
+
+    Auth.#instance = this;
+
+    this.#uuid = uuidv4();
+  }
+
+  get uuid() {
+    return this.#uuid;
+  }
+
+  get onAuthStateChangedObservers() {
+    return this.#onAuthStateChangedObservers;
+  }
+
+  dispose() {
+    this.#onAuthStateChangedObservers = [];
+  }
+
+  onAuthStateChanged(observer) {
+    this.#onAuthStateChangedObservers.push(observer);
+
+    return () => {
+      this.#onAuthStateChangedObservers = this.#onAuthStateChangedObservers.filter(
+        (observer) => observer !== observer,
+      );
+    };
+  }
+
+  async signInAnonymously() {
+    const user = AuthConstants.anonymousUser1;
+
+    this.#onAuthStateChangedObservers.forEach((observer) => observer(user));
+
+    return Promise.resolve(user);
+  }
+}
+
+export default Auth;