chore: prefer const (#865)

packages/runtime-core/__tests__/apiWatch.spec.ts

@@ -112,8 +112,8 @@ describe('api: watch', () => {
     let dummy
     watch([() => state.count, status] as const, (vals, oldVals) => {
       dummy = [vals, oldVals]
-      let [count] = vals
-      let [, oldStatus] = oldVals
+      const [count] = vals
+      const [, oldStatus] = oldVals
       // assert types
       count + 1
       oldStatus === true
@@ -412,7 +412,7 @@ describe('api: watch', () => {
 
   it('warn and not respect deep option when using effect', async () => {
     const arr = ref([1, [2]])
-    let spy = jest.fn()
+    const spy = jest.fn()
     watchEffect(
       () => {
         spy()