[Cases] Test case_view_activity.test.tsx suite (elastic#160639)

## Summary

Fixes flaky tests in
`x-pack/plugins/cases/public/components/case_view/components/case_view_activity.test.tsx`

Successful runs:

-
elastic@a7a19c1
(40 times)
-
elastic@f5e0645
(45 times)
-
elastic@d9838cc
(40 times)

Fixes: elastic#151981,
elastic#151979,
elastic#151980,
elastic#152209,
elastic#152207,
elastic#152208,
elastic#152203,
elastic#152203,
elastic#152201,
elastic#152206,
elastic#152202,
elastic#152204,
elastic#152200

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: Kibana Machine <[email protected]>
(cherry picked from commit 6349724)