Add approvals store spec.

spec/javascripts/approvals/approvals_store_spec.js.es6

+//= require jquery
+//= require vue
+//= require vue-resource
+//= require merge_request_widget/approvals/stores/approvals_store
+
+$.rails = {
+  csrfToken() {},
+};
+
+(() => {
+  // store constructor reference, to overwrite ApprovalStore singleton
+  gl.ApprovalsConstructor = gl.ApprovalsStore;
+
+  // stand in for promise returned by api calls
+  const mockThenable = {
+    then() {
+      return {
+        catch() {},
+      };
+    },
+    catch() {},
+  };
+
+  const mockRootStore = {
+    data: {},
+    dataset: {
+      endpoint: 'gitlab/myendpoint/',
+    },
+    assignToData(key, val) {
+      return { key, val };
+    },
+  };
+
+  describe('Approvals Store', function () {
+    beforeEach(function () {
+      this.rootStore = mockRootStore;
+      this.approvalsStore = new gl.ApprovalsConstructor(this.rootStore);
+    });
+
+    it('should define all needed approval api calls', function () {
+      expect(this.approvalsStore.fetch).toBeDefined();
+      expect(this.approvalsStore.approve).toBeDefined();
+      expect(this.approvalsStore.unapprove).toBeDefined();
+    });
+
+    it('should only init the store once', function () {
+      spyOn(this.approvalsStore, 'fetch').and.callFake(() => mockThenable);
+
+      this.approvalsStore.initStoreOnce();
+      this.approvalsStore.initStoreOnce();
+      this.approvalsStore.initStoreOnce();
+
+      expect(this.approvalsStore.fetch.calls.count()).toBe(1);
+    });
+
+    it('should be able to write to the rootStore', function () {
+      const dataToStore = { myMockData: 'string' };
+
+      spyOn(this.rootStore, 'assignToData');
+
+      this.approvalsStore.assignToRootStore(dataToStore);
+
+      expect(this.rootStore.assignToData).toHaveBeenCalled();
+      expect(this.rootStore.assignToData).toHaveBeenCalledWith('approvals', dataToStore);
+    });
+  });
+})(window.gl || (window.gl = {}));