Only display active roles in CaseParticipantsTab component, add Vitest, and unit test(s)

[wssheldon]

This PR prevents renounced roles from being displayed in the CaseParticipantsTab component. Previously, when someone has frequently changed roles, their roles will appear as `('Reporter', 'Assignee', 'Participant', 'Assignee'), for example. We only display roles that haven't been renounced and add support for testing Vue 2 components with Vitest [0]. Finally, we add a unit test for this specific case and others for the CaseParticipantsTab component.

 ✓ tests/participants-tab.spec.js (2)

 Test Files  1 passed (1)
      Tests  2 passed (2)
   Start at  08:36:41
   Duration  778ms (transform 211ms, setup 0ms, collect 191ms, tests 14ms)


 PASS  Waiting for file changes...
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[0] https://vitest.dev/

[wssheldon]

@kevgliss @mvilanova Any opinions on where the tests for front-end should be stored? Here they are stored in the static/ dir and E2E tests are currently in the gitroot tests/ dir. Which do you prefer?