fix: include running Jest tests as part of CI workflow (#531)

I've included `--coverage` though we've not actually got a min. coverage
target set so it won't enforce anything, but I think that's a separate
thing.

Resolves #422

variants/github_actions_ci/workflows/ci.yml.tt

@@ -53,6 +53,7 @@ jobs:
       <%- end -%>
       - run: yarn run js-lint
       - run: yarn run format-check
+      - run: yarn run test --coverage
   ruby_based_checks:
     permissions:
       contents: read