`watch` mounting option should overwrite option on same component

[lmiller1990]

Version

1.0.0-beta.30

Reproduction link

https://i-included-a-repro.com

Steps to reproduce

repro

import { mount, shallowMount } from '@vue/test-utils'

const Comp = {
  template: '<div />',
  props: ['myProp'],
  watch: {
    myProp: {
      handler(val, oldVal) {
        console.log('Original watcher triggered!')
      },
      deep: true
    }
  }
}

test('Both watchers trigger', (done) => {
  const wrapper = mount(Comp, {
    propsData: {
      myProp: 'foo'
    },
    watch: {
      myProp: {
        handler(val, oldVal) {
          console.log('Overridden watcher triggered!')
        },
        deep: true
      }
    }
  })

  wrapper.setProps({ myProp: 'ok' })

  setTimeout(() => {
    done()
  }, 100)
})

What is expected?

Only the mounting options watcher should do anything.

What is actually happening?

Both watchers are triggered.

---

This is because of this line here which uses Vue's watcher merge strategy, which is to create an array of watchers. That code is in Vue core, found here.