Add test coverage for app/assets/javascripts/vue_merge_request_widget/components/checks/constants.js

From !158773 (comment 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