Add more tests for calendar contribution

## What does this MR do?
Add more tests for calendar contribution

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
Improve test coverage for calendar

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #22347

See merge request !6421