Fix MR widget tests to play well with Vue.js 3

What does this MR do and why?

This MR fixes our MR widget tests to play well with Vue.js 3

We're storing components inside Vue.observable so we should take extra steps to avoid them becoming reactive (Vue.js 3 does not like it)

Additionally it complains about missing prop 😄

Screenshots or screen recordings

N/A

How to set up and validate locally

VUE_VERSION=3 yarn jest ee/spec/frontend/vue_merge_request_widget/ee_mr_widget_options_spec.js

should pass

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.