test(reactivity/effect): add test for lazy option (vuejs#179)

Author: tangdaohai

packages/reactivity/__tests__/effect.spec.ts

@@ -505,6 +505,18 @@ describe('reactivity/effect', () => {
     expect(dummy).toBe(1)
   })
 
+  it('lazy', () => {
+    const obj = reactive({ foo: 1 })
+    let dummy
+    const runner = effect(() => (dummy = obj.foo), { lazy: true })
+    expect(dummy).toBe(undefined)
+
+    expect(runner()).toBe(1)
+    expect(dummy).toBe(1)
+    obj.foo = 2
+    expect(dummy).toBe(2)
+  })
+
   it('scheduler', () => {
     let runner: any, dummy
     const scheduler = jest.fn(_runner => {