Unit tests for Usergroup column parsing

[enjeck]

it would be nice to also have a test for the selection column, considering we have https://github.com/nextcloud/tables/blob/main/tests/unit/Service/ColumnTypes/TextLinkBusinessTest.php too

Originally posted by @enjeck in #1887 (review)

[AIlkiv]

To avoid forgetting, it is important to check the cases:

1. parseDisplayValue for user
2. parseDisplayValue for team
3. parseDisplayValue for group
4. when there are several users/teams/groups
5. when there are several teams and one has commas in its name (so that if the separator between elements is a comma, the team name must be additionally escaped).