test: move mockWarn into setup files

packages/reactivity/__tests__/collections/Map.spec.ts

@@ -1,10 +1,7 @@
 import { reactive, effect, toRaw, isReactive } from '../../src'
-import { mockWarn } from '@vue/shared'
 
 describe('reactivity/collections', () => {
   describe('Map', () => {
-    mockWarn()
-
     test('instanceof', () => {
       const original = new Map()
       const observed = reactive(original)

packages/reactivity/__tests__/collections/Set.spec.ts

@@ -1,10 +1,7 @@
 import { reactive, effect, isReactive, toRaw } from '../../src'
-import { mockWarn } from '@vue/shared'
 
 describe('reactivity/collections', () => {
   describe('Set', () => {
-    mockWarn()
-
     it('instanceof', () => {
       const original = new Set()
       const observed = reactive(original)

packages/reactivity/__tests__/computed.spec.ts

@@ -7,11 +7,8 @@ import {
   WritableComputedRef,
   isReadonly
 } from '../src'
-import { mockWarn } from '@vue/shared'
 
 describe('reactivity/computed', () => {
-  mockWarn()
-
   it('should return updated value', () => {
     const value = reactive<{ foo?: number }>({})
     const cValue = computed(() => value.foo)

packages/reactivity/__tests__/reactive.spec.ts

@@ -1,11 +1,8 @@
 import { ref, isRef } from '../src/ref'
 import { reactive, isReactive, toRaw, markRaw } from '../src/reactive'
-import { mockWarn } from '@vue/shared'
 import { computed } from '../src/computed'
 
 describe('reactivity/reactive', () => {
-  mockWarn()
-
   test('Object', () => {
     const original = { foo: 1 }
     const observed = reactive(original)

packages/reactivity/__tests__/readonly.spec.ts

@@ -10,7 +10,6 @@ import {
   shallowReadonly,
   isProxy
 } from '../src'
-import { mockWarn } from '@vue/shared'
 
 /**
  * @see https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html
@@ -18,8 +17,6 @@ import { mockWarn } from '@vue/shared'
 type Writable<T> = { -readonly [P in keyof T]: T[P] }
 
 describe('reactivity/readonly', () => {
-  mockWarn()
-
   describe('Object', () => {
     it('should make nested values readonly', () => {
       const original = { foo: 1, bar: { baz: 2 } }