fix(store): provide the same instance of MockStore (#2381)

Closes #2362

Author: lacolaco

modules/store/testing/spec/mock_store.spec.ts

@@ -63,6 +63,12 @@ describe('Mock Store', () => {
     mockStore.resetSelectors();
   });
 
+  it('should provide the same instance with Store and MockStore', () => {
+    const fromStore = TestBed.get(Store);
+    const fromMockStore = TestBed.get(MockStore);
+    expect(fromStore).toBe(fromMockStore);
+  });
+
   it('should set the initial state to a mocked one', (done: DoneFn) => {
     const fixedState = {
       counter1: 17,

modules/store/testing/src/testing.ts

@@ -28,7 +28,7 @@ export function provideMockStore<T = any>(
     { provide: MOCK_SELECTORS, useValue: config.selectors },
     { provide: StateObservable, useClass: MockState },
     { provide: ReducerManager, useClass: MockReducerManager },
-    { provide: Store, useClass: MockStore },
+    { provide: Store, useExisting: MockStore },
   ];
 }