Fix spec violations in spec/frontend/vue_merge_request_widget/mr_widget_options_spec.js

Problem description

The spec file spec/frontend/vue_merge_request_widget/mr_widget_options_spec.js contains technical debt and/or antipatterns that need to be fixed. This is a prerequisite for updating our codebase to Vue3.

Proposal

The following issues have been identified in this spec file:

• overriding method behavior
• .setData() usage

Please see the related epic for more details on how these violations can be addressed.

Note to self: This file is large, so double check https://gitlab-org.gitlab.io/frontend/playground/vue-compat-scanner/ to stay on task and fix only what needs to be fixed