refactor: move mockWarn utility to @vue/shared

close #652

packages/reactivity/__tests__/computed.spec.ts

@@ -6,7 +6,7 @@ import {
   ref,
   WritableComputedRef
 } from '../src'
-import { mockWarn } from '@vue/runtime-test'
+import { mockWarn } from '@vue/shared'
 
 describe('reactivity/computed', () => {
   mockWarn()

packages/reactivity/__tests__/reactive.spec.ts

@@ -1,6 +1,6 @@
 import { ref, isRef } from '../src/ref'
 import { reactive, isReactive, toRaw, markNonReactive } from '../src/reactive'
-import { mockWarn } from '@vue/runtime-test'
+import { mockWarn } from '@vue/shared'
 import { computed } from '../src/computed'
 
 describe('reactivity/reactive', () => {

packages/reactivity/__tests__/readonly.spec.ts

@@ -12,7 +12,7 @@ import {
   ref,
   shallowReadonly
 } from '../src'
-import { mockWarn } from '@vue/runtime-test'
+import { mockWarn } from '@vue/shared'
 
 /**
  * @see https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html