test: move mockWarn into setup files

packages/runtime-core/__tests__/helpers/renderSlot.spec.ts

@@ -1,9 +1,7 @@
 import { renderSlot } from '../../src/helpers/renderSlot'
 import { h } from '../../src/h'
-import { mockWarn } from '@vue/shared'
 
 describe('renderSlot', () => {
-  mockWarn()
   it('should render slot', () => {
     let child
     const vnode = renderSlot(

packages/runtime-core/__tests__/helpers/resolveAssets.spec.ts

@@ -12,11 +12,8 @@ import {
   Comment,
   VNode
 } from '@vue/runtime-test'
-import { mockWarn } from '@vue/shared'
 
 describe('resolveAssets', () => {
-  mockWarn()
-
   test('should work', () => {
     const FooBar = () => null
     const BarBaz = { mounted: () => null }

packages/runtime-core/__tests__/helpers/toHandlers.spec.ts

@@ -1,9 +1,6 @@
 import { toHandlers } from '../../src/helpers/toHandlers'
-import { mockWarn } from '@vue/shared'
 
 describe('toHandlers', () => {
-  mockWarn()
-
   it('should not accept non-objects', () => {
     toHandlers(null as any)
     toHandlers(undefined as any)