test(): add mixpanel test back

test/plugins/mixpanel.spec.ts

@@ -0,0 +1,28 @@
+import {Mixpanel} from '../../src/plugins/mixpanel';
+declare const window: any;
+
+window.mixpanel = {
+  people: {
+    identify: (args, success, error) => success('Success')
+  }
+};
+
+describe('Mixpanel', () => {
+
+  it('should return MixpanelPeople', () => {
+    expect(Mixpanel.people).toBeDefined();
+    expect(Mixpanel.people.identify).toBeDefined();
+  });
+
+  it('should call a method of MixpanelPeople', (done) => {
+    const spy = spyOn(window.mixpanel.people, 'identify').and.callThrough();
+    Mixpanel.people.identify('veryDistinctSuchIdVeryWow')
+      .then(result => {
+        expect(result).toEqual('Success');
+        done();
+      });
+    expect(spy.calls.mostRecent().args[0]).toEqual('veryDistinctSuchIdVeryWow');
+    expect(window.mixpanel.people.identify).toHaveBeenCalled();
+  });
+
+});