Refactor merge request widget state tests

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The following discussion from !36624 (merged) should be addressed:

• @psimyn started a discussion:

  to make these assertions not depend on order (which could lead to confusing test failures and depends on function internals), could we move context/data/bound to beforeEach and each of these to a separate spec (or use it.each)?

  Feel free to followup issue (or ignore) if you want since that's a bunch of extra changes.

In short: we should refactor the entire large test in spec/frontend/vue_mr_widget/stores/get_state_key_spec.js into - probably - a table of it.each so that their order doesn't matter.