FirebaseStub: added more unit tests related to functions

__mocks__/__tests__/gatsby-plugin-firebase/functions.test.js

@@ -0,0 +1,19 @@
+import FirebaseStub from '../../gatsby-plugin-firebase';
+
+test('reuses existing Functions instance', () => {
+  const functions1 = FirebaseStub.functions();
+  const functions2 = FirebaseStub.functions();
+
+  expect(functions1.uuid).toBeTruthy();
+  expect(functions2.uuid).toBeTruthy();
+  expect(functions1.uuid).toEqual(functions2.uuid);
+});
+
+test('deleteUser function returns true', async () => {
+  const deleteUser = FirebaseStub.functions().httpsCallable('deleteUser');
+
+  const result = await deleteUser();
+
+  expect(result).toBeTruthy();
+  expect(result.data).toEqual(true);
+});