Add more tests for marker categories

While reviewing #3256, I realized that we don't have enough tests related to marker categories. And that's why we also don't have fixtures to add markers with given categories.
We should add more tests related to marker categories. For example, for the marker chart marker bucket sorting. It has some special handling for `Test` and `Other` categories that we can test.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-374)