Refactor unit test report to use namespaced vuex module

What does this MR do and why?

For #363076 (closed)

This MR refactors the vuex store for the unit test report (the Tests tab on the pipeline page) to make it into a namespaced module in preparation for #360794 (closed).

• The Vuex store should be created in the app's entry point rather than its store/index.js file.

  • Move Vue.use(Vuex) to the app's entry point.
  • store/index.js only returns the store's options.
  • The store is instantiated with new Vuex.Store in the entry point.

• The stores should be converted to namespaced modules.

  • In store/index.js, place all options into a namespaced module.
  • Optionally, move state, actions, mutations and getters files to a sub-directory.
  • Review all map* helper usages so that they properly map to the module rather than the root.

• The app's root component should register its module(s) in the created hook. This can be done with the registerModule method. More on this in https://vuex.vuejs.org/guide/modules.html#dynamic-module-registration.

• app's entry point: app/assets/javascripts/pipelines/pipeline_test_details.js

• store index: app/assets/javascripts/pipelines/stores/test_reports/index.js

• app's root component: app/assets/javascripts/pipelines/components/test_reports/test_reports.vue

Screenshots or screen recordings

No visual changes are expected, but here's the tab:

How to set up and validate locally

0. git checkout 363076-unit-test-report-to-vuex-module
1. find/clone a project with unit test reports
2. run a pipeline in the project to generate a test report for the tab to display
3. navigate to the pipeline, click on the Tests tab
4. everything should work exactly the same as it does on master - the test summary should load, you should be able to click on each suite and see its tests, there should be a spinner when the suite is loading, there should be an empty state if there are no tests, etc.

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.