Fix flaky spec in QuickActions::InterpretService#execute

The test was attempting to test whether /assign @userA @userB would only assign an issue or merge request to userA. However, this would fail occasionally because the reference extractor does not guarantee ordering with extracted Markdown references since a database lookup is performed (e.g. User.where) without an ORDER BY.

We adapt the test to check that only one of the two possible users is assigned since the behavior is ambiguous when multiple assignees is not supported, and it would take some work with some performance implications to ensure the first mentioned user were assigned.

Closes #27989 (closed)