FirebaseStub: added more unit tests related to auth

__mocks__/__tests__/gatsby-plugin-firebase/auth.signOut.test.js

@@ -0,0 +1,29 @@
+import FirebaseStub from '../../gatsby-plugin-firebase';
+
+test('sets current user to null', async () => {
+  await FirebaseStub.auth().signInAnonymously();
+
+  await FirebaseStub.auth().signOut();
+
+  const { currentUser } = FirebaseStub.auth();
+  expect(currentUser).toBeNull();
+});
+
+test('calls onAuthStateChanged observer with null', async () => {
+  let user = null;
+  let error = null;
+  FirebaseStub.auth().onAuthStateChanged(
+    (_user) => {
+      user = _user;
+    },
+    (_error) => {
+      error = _error;
+    },
+  );
+  await FirebaseStub.auth().signInAnonymously();
+
+  await FirebaseStub.auth().signOut();
+
+  expect(user).toBeNull();
+  expect(error).toBeNull();
+});