[v2] rewrite proxy for setupTest

[ricardogobbosouza]

describe('module', () => {
  const { nuxt } = setupTest({
    __dirname,
    fixture: 'example'
  })

  test('render',() => {
    // nuxt undefined
  })
})

[pi0]

This won't be necessary anymore with #51. One can use getNuxt()