chore(vue3): explicitly unstub root component in test

What does this MR do?

@vue/test-utils v1 has a limitation - when you're using shallowMount it is not stubbing the component, located at root.

So if you have following template (for component named GlFormGroup):

<template>
  <b-form-group>
    <div><b-some-component></b-some-component></div>
  </b-form-group>
</template>

in @vue/test-utils@1 only <b-some-component> will be stubbed:

flowchart LR
GlFormGroup:::unstubbed-->BFormGroup:::unstubbed-->div-->BSomeComponent:::stubbed

classDef unstubbed fill:lightgreen,stroke:black,stroke-width:4px
classDef stubbed fill:pink,stroke:black,stroke-width: 4px, stroke-dasharray: 20 5

This is due to limitations of extremely hacky createElement patching.

In @vue/test-utils@2 situation is more consistent and expected:

flowchart LR
GlFormGroup:::unstubbed-->BFormGroup:::stubbed-->div-->BSomeComponent:::stubbed

classDef unstubbed fill:lightgreen,stroke:black,stroke-width:4px
classDef stubbed fill:pink,stroke:black,stroke-width: 4px, stroke-dasharray: 20 5

In order to support future migration to @vue/compat we're explicitly unstubbing BFormGroup in test

This is forward-compatible change to support @vue/compat #1981 (closed)

Does this MR meet the acceptance criteria?

Conformity

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• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
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• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
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• Added the ~"component:*" label(s) if applicable.