Adds Vuex store to handle the data for tests reports in MR widget

What does this MR do?

Adds Vuex store to handle the data for tests reports in MR widget

It will consume and render the following data:

{
   "summary":{
      "total":14,
      "resolved":0,
      "failed":7
   },
   "suites":[
      {
         "name":"build:linux",
         "summary":{
            "total":2,
            "resolved":0,
            "failed":1
         },
         "new_failures":[
            {
               "name":"StringHelper#concatenate when a is git and b is lab returns summary",
               "execution_time":"0.009235",
               "system_output":"Failure/Error: is_expected.to eq('gitlab')\n\n  expected: \"gitlab\"\n       got: \"git  lab\"\n\n  (compared using ==)\n./spec/string_helper_spec.rb:12:in `block (4 levels) in \u003ctop (required)\u003e'"
            }
         ],
         "resolved_failures":[

         ],
         "existing_failures":[

         ]
      },

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

#45318 (closed)

https://gitlab.com/gitlab-org/gitlab-ce/issues/49375