Fix spec violations in vuex module provider spec

What does this MR do and why?

Removes Vue.extend from Vuex module provider.

Screenshots or screen recordings

N/A

How to set up and validate locally

npm run jest -- -- spec/frontend/vue_shared/components/vuex_module_provider_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #391121 (closed)