fix: disable stubs for built-in components on shallow

[freakzlike]

fix #929

I came across an equal issue when using shallow: true but still want to use the real teleport. This ensures to use the real component when using

const wrapper = mount(Comp, {
  shallow: true,
  global: {
    stubs: {
      transition: false,
      teleport: false,
      'transition-group': false
    }
  }
})

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[cexbrayat]

LGTM, I'll let @lmiller1990 take a look and merge

[lmiller1990]

Seems fine - I think it's weird we have so much special code around shallow - I generally just use mount for this reason 🤷

[freakzlike]

You are right. There are also quiet some open issues related to shallow and stubs. I think some things can be simplified or maybe ( for the future) VTU only offers some API for plugins.