Add test coverage for app/assets/javascripts/vue_merge_request_widget/components/checks/constants.js
From https://gitlab.com/gitlab-org/gitlab/-/merge_requests/158773#note_2032623652:
> I was a little surprised to see that no ~frontend tests are affected by this change, it seems we probably dont have jest and feature tests for all the merge checks / mergeability states.
>
> I think it would be great (probably a follow-up) to expand the tests, perhaps by adding additional feature tests. The closest spot I could find is probably https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/spec/features/merge_trains, but it might make sense to double check with a relevant ~frontend engineer who is more familiar with this area.
>
> Otherwise LGTM and i'll set MWPS since it looks like we have all other approvals.
cc @ekigbo
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